Reexamination of Color 

Vision Standards, Part 

III: Analysis of the 

Effects of Color Vision 

Deficiencies in Using 

ATC Displays 

Jing Xing 

Civil Aerospace Medical Institute

Federal Aviation Administration

Oklahoma City, OK 73125

May 2006

Final Report

DOT/FAA/AM-06/11

Office of Aerospace Medicine

Washington, DC 20591
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of information exchange. The United States Government 

assumes no liability for the contents thereof.

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This publication and all Office of Aerospace Medicine 

technical reports are available in full-text from the Civil 

Aerospace Medical Institute’s publications Web site: 

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i

Technical Report Documentation Page 

1. Report No. 2. Government Accession No. 3. Recipient's Catalog No. 

DOT/FAA/AM-06/11 

4. Title and Subtitle 5. Report Date 

Reexamination of Color Vision Standards, Part III: Analysis of the May 2006 

Effects of Color Vision Deficiencies in Using ATC Displays 6. Performing Organization Code 

 

7. Author(s) 8. Performing Organization Report No. 

Xing J 

9. Performing Organization Name and Address 10. Work Unit No. (TRAIS) 

FAA Civil Aerospace Medical Institute 

P.O. Box 25082 11. Contract or Grant No. 

Oklahoma City, OK 73125 

12. Sponsoring Agency name and Address 13. Type of Report and Period Covered 

Office of Aerospace Medicine 

Federal Aviation Administration 

800 Independence Ave., S.W. 

Washington, DC 20591 14. Sponsoring Agency Code 

15. Supplemental Notes 

Work was accomplished under approved task AM-HRRD522 

16. Abstract 

The purpose of this report is to assess the effect of color use in air traffic control (ATC) displays for users who have 

color vision deficiencies, denoted as color deficient (CD). At present, color is extensively used in many ATC displays, 

while the color vision standard used by the Federal Aviation Administration (FAA) allows certain types of CDs to 

enter the ATC workforce. Many guidelines for color use in visual displays state that color use should be accompanied 

with achromatic redundant cues to avoid misinterpretation by CD users. However, little has been documented in 

guidelines about the effect of redundant cues. Therefore, it is necessary to understand how CD personnel use colorcoded information in displays and whether redundant cues are helpful. Previously, we collected data about color use in 

displays from many ATC facilities. We also developed computational algorithms that could assess the effects of color 

vision deficiencies on the performance of color-related ATC tasks. The algorithms compared the effectiveness of using 

color-coded information between observers with normal color vision and CDs. The algorithms also considered the 

effectiveness of redundant visual cues relative to colors. In this report, we applied the algorithms to six ATC displays 

to estimate their efficient use by CDs. The main findings included the following: 1) Critical color-coded information 

may not capture the attention of CDs in many applications; 2) There are instances where CDs may not reliably identify 

types of information that are encoded in colors; and 3) In many instances color use makes text reading slower and less 

accurate for CDs. These results indicate that CDs may not be able to use color displays as efficiently as users with 

normal vision. In addition, we identified the situations where no redundant cues were used for task-critical color 

usages. Moreover, we estimated that most redundant cues were not as effective as color or not effective at all for the 

given task. 

 

17. Key Words 18. Distribution Statement 

 

 

Air Traffic Control, Displays, Color Deficiency, 

Algorithms, Human Factors Evaluation.

Document is available to the public through the 

Defense Technical Information Center, Ft. Belvior, VA 

22060; and the National Technical Information 

Service, Springfield, VA 22161 

19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price 

Unclassified Unclassified 22 

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized

iii

ACKNOWLEDGMENTS

We thank Mr. Keith Gunnell for his effort in coordinating facility visits for us. This project could 

not have been done without his help. We thank Drs. Carol Manning, Larry Bailey, David Schroeder, 

and Steve Cooley for their valuable comments on the manuscript. We also express our appreciation of the support from Dino Piccione and Steve Cooley. We thank Janine King for her enormous 

help in editing an early version of the manuscript. We appreciate all the FAA reviewers and editors 

whose efforts greatly improved this report. We thank the support from the following ATC facilities: 

ZDC, ZNY, ZNC, PHL Tower, DC National Tower, SF Tower, New York TRACON, Potomac 

TRACON, and PHL TRACON.

1

Reexamination of Color Vision Standards, Part III: Analysis of

the Effects of Color Vision Deficiencies in Using ATC Displays

INTRODUCTION

About 8-10% of all males have various degrees of color 

vision deficiency, which makes them unable to discriminate between colors that appear different to people with 

normal color vision. This report is a continuing effort to 

reexamine the FAA’s color vision standards and focuses on 

the effect of color vision deficiencies on using air traffic 

control (ATC) displays. In our previous studies (Xing & 

Schroeder, 2005a,b), we observed color use at nine ATC 

facilities: three airport control towers, three Terminal 

Radar Approach Control (TRACONS) facilities, and 

three en route ATC centers. The results revealed extensive 

color use of ATC displays. We then developed a set of 

algorithms to compute the effect of color vision deficiencies. In the current report, we applied the algorithms to 

six major ATC displays, and we analyzed the effects of 

color use on task performance of air traffic controllers 

who have color deficiencies. 

The color vision standard used by the FAA may not 

adequately cover the scope of color use in current ATC displays (Xing & Schroeder, 2005a). To understand whether 

the standard is sufficient for current task requirements 

associated with color use, it is essential to understand the 

effect of color vision deficiencies on using color displays. 

Given that many color displays are being used in ATC 

facilities, it is not practical to run a study by recruiting 

a group of color-deficient controllers to perform ATC 

tasks using all of the current displays. Therefore, in the 

previous report, we developed a set of computational 

algorithms to assess the potential effect of color on the 

performance of individuals with color vision deficiencies, 

denoted as color deficient (CD), relative to personnel with 

normal color vision, denoted as color-normal (CN). The 

algorithms used a computational method that simulated 

colors as seen by deuteranopes and protanopes, the two 

major types of CDs (Brettel, Vienot, & Mollon, 1997; 

Vienot, Brettel, & Mollon, 1999). The algorithms allowed 

us to assess the efficiency of color displays for CDs by 

calculating the effectiveness of color for CDs in a given 

task. By applying the algorithms to existing ATC displays, 

we can identify the situations where color use could be a 

potential problem for CD controllers, and high-fidelity 

experiments can be focused on those situations to examine 

the exact effects of color on CDs’ task performances. 

Our previous study (Xing & Schroeder, 2005a) has 

shown that colors on ATC displays are used for three 

purposes: 1) to capture attention; salient colors are often 

chosen to encode information that needs to be attended 

to immediately, such as an alert or emergency; 2) to 

identify categories of information so that searching for 

specific information in a complex image can be done 

more effectively; in this case, each color is associated with 

a distinctive meaning; and 3) to segment complex images 

in a display into distinctive groups so that information 

belonging to the same category can be organized together. 

In this application, color does not have inherent meanings. 

Therefore, we use three terms to refer to color-related 

tasks throughout this report: attention, identification, 

and segmentation. In addition, since reading text is an 

important part of many ATC tasks, and the efficiency 

of reading is affected by color use, we also analyzed the 

effect of color in text reading. 

This report describes our analysis of the effect of 

color vision deficiencies on the use of ATC displays. 

We analyzed the effect for several primary displays with 

which controllers handle aircraft, and several widely 

used supporting displays from which controllers acquire 

additional information for decision-making. Finally, we 

conducted an analysis of the overall effects of color vision deficiencies on ATC task performance. The results 

provide insights into the color vision deficit problem in 

using existing ATC displays and suggest the need for 

further task analysis and experimental studies.

METHODS

Data Acquisition

The data on color use in this study are mainly acquired 

from the pictures of ATC displays we took during our 

facility observations (Xing & Schroeder, 2005a). For 

circumstances where the resolution of the digital pictures 

was low or where some colors were not available at the 

time of observations (e.g., color-coded weather information), we used other sources of information to re-create 

the images of color displays. Those sources included: 1) 

color descriptions recorded during facility observations; 

2) color specifications obtained from manufacturers; and 

3) interface descriptions of the displays. 
2

ATC displays

We analyzed the effects of color vision deficiencies for 

all the primary displays currently used by operational 

controllers to control aircraft. The primary display in 

en route ATC centers is Display System Replacement 

(DSR) for inland airspace or the Advanced Technologies 

and Oceanic Procedures (ATOP) for oceanic airspace. 

Unfortunately, data about ATOP were not available 

because it was not in full operation at the time of our 

facility observation. The primary displays for TRACONs 

are Standard Terminal Automation Replacement System 

(STARS) and Automated Radar Terminal System (ARTS) 

Color Display (ACD). For airport control towers, while 

controllers do not use radar displays to control aircraft, 

it is crucial for controllers to acquire accurate weather 

and traffic information in a timely manner. Thus we 

analyzed the Integrated Terminal Weather System (ITWS) 

being used in many control towers. In addition, the User 

Request Evaluation Tool (URET) and Traffic Management Advisor (TMA) are two major support tools for en 

route controllers to acquire additional information. Since 

URET and TMA are being used or are scheduled to be 

deployed to many en route facilities, they were chosen 

to represent supporting tools for our analysis. 

Review of the algorithms 

We previously developed a set of algorithms to assess 

the effect of color vision deficiencies by considering two 

factors: achromatic redundant cues and color for CDs. 

Presumably, if either works, then CDs should have no 

problem using color displays. Hence, the analysis of color 

use included determining the effect of redundant cues 

relative to colors and computing the effect of colors for 

CDs relative to CNs. Taking the two factors together, 

the algorithms provide an overall assessment about how 

CDs use color displays.

Effect of redundant cues relative to color

When achromatic visual features are used to identify 

information in addition to color, those features are considered to be redundant cues. While it is desirable that 

CDs can use redundant cues to acquire color-coded 

information, the effect of redundant cues may not be the 

same as that of color for a given task. We synthesized the 

data in the literature about the effectiveness of achromatic 

visual features relative to color. For attention tasks, flashing 

and brightness are equivalent to or more effective than 

color, while shape and text are not effective in general 

(Christ, 1975). A sudden onset of color and luminance 

can also draw attention under some circumstances (Yantis 

& Jonides, 1996). For identification tasks, color is superior to achromatic cues such as location, shapes, and 

text, while luminance and size are not effective (Young 

& Nagy, 2003; Christ, 1975). Some ATC displays 

employed another type of redundant cue: Although no 

achromatic features are visually present along with color 

use, color-coded information can be inferred from other 

parts of the displays. We refer to such cues as “inference” 

redundant cues. They are less effective than color in terms 

of the time and the amount of brain processing required 

to comprehend the information.

Effects of color for CDs relative to CNs

The effects of color in attention, identification, and text 

readability may depend on the chromaticity, luminance, or 

both. In general, the effectiveness of color increases with 

the luminance and /or chromaticity difference between 

the color-coded targets and other materials in a display. 

In the preceding report, we synthesized the data in the 

literature and determined the threshold of luminance 

or chromaticity difference, below which the use of color 

was not effective for a given task. The results are summarized as follows:

•	 The effect of color in attention tasks depends on visual 

conspicuity of the target, which, in turn, depends 

on chromaticity and luminance differences between 

the target and other materials presented in the visual 

field. Those materials are denoted as distracters. The 

threshold point for luminance difference is 20cd/cm2. 

The threshold point for chromaticity difference is 0.24 

in CIE (Commission Internationale de l’Eclairage) 

chromaticity coordinates (Nagy & Sanchez, 1990; 

1992). 

•	 The effect of color in identification tasks depends on 

the chromaticity differences between colors and how 

well the colors can be named. The threshold difference for identification is 0.036 in CIE chromaticity 

coordinates. Below the threshold, two colors cannot 

be named distinctively (Smallman & Boynton, 1990; 

Boynton, MacLaury, & Uchikawa, 1989).

•	 Text readability is defined as the property that permits 

an observer to read text easily on a screen irrespective 

of the text meaning. It mainly depends on the luminance contrast between the text and background colors 

(Legge, Parish, Luebker, & Wurm, 1990). Readability is 

typically measured as the time required to find and read 

a given text or the number of words read per minute. 

Many experiments have demonstrated that reading 

speed decreases as the text contrast decreases. Below 

20% contrast, reading speed slows down much more 

rapidly and is significantly below the normal reading 

speed (Legge et al., 1990; Scharff & Ahumada, 2002). 

Thus, the threshold contrast for reading is usually 

taken as 20%. 
3

Algorithms

We applied the algorithms to ATC displays using the 

following steps:

1) Determine situations where color-coding is associated 

with a task. 

2) Determine achromatic redundant cues. 

3) Determine the effectiveness of redundant cues relative 

to that of color.

4) Determine color specifications for the targets, distracters, and background. The specifications are the digital 

values of the three monitor color channels, denoted 

as r for red, g for green, and b for blue.

5) Determine the default color, which is the dominant 

color of distracters.

6) Determine the effectiveness of color for CDs relative 

to CNs.

7) Combine the effect of redundant cues and the effect 

of color for CDs to determine the overall effectiveness 

of color use.

In the previous report (Xing & Schroeder, 2005b), 

we classified the effect of CDs into three levels: “E” was 

denoted for the situation where color was equally or 

more effective for CDs than for CNs; “L” represented 

the situation where color was less effective for CDs than 

for CNs; and “NE” meant that color was not effective for 

CDs. Similarly, we categorized the effect of redundant 

cues into three levels: “E” was denoted for the situation 

where a redundant cue was equally or more effective than 

color; “L” represented the situation where a redundant 

cue was less effective than color; and “NE” meant that 

a redundant cue was not effective. We continue to use 

these three-level scales to describe the results in this report. 

In addition, “NA” is used to denote situations where a 

redundant cue is “not available.”

RESULTS

We were mainly concerned with how color use affected 

ATC task performance for CDs; thus, the analysis in this 

section focused only on colors associated with important 

ATC tasks. Specifically, only the situations where color 

was used as the primary cue for attention and identification were considered in the analysis. 

Most ATC displays are complex, and information is 

not static. To make a systematic analysis and avoid missing 

important color usages, we analyzed color use by functional 

components of a given display. Typical components in 

ATC displays included datablocks, targets, boundaries, 

menus, weather, etc. 

Automated Radar Terminal System Color Display 

(ACD)

ACD is a primary display used by operational controllers at selected TRACON and tower facilities. It 

provides controllers with a stable, high-resolution color 

display of aircraft positions and a graphical user interface 

for control functions. Among the display components, 

datablocks and weather use color-coding for task-critical 

information.

Datablocks 

Datablocks are displayed across the central area of an 

ACD screen. The background color is dark blue. While 

controllers can adjust the background luminance, they 

usually leave it very dark, close to black. Most datablocks 

are either white or green. White identifies datablocks 

owned by a controller, and green identifies those not 

owned. 

When the system detects a potential conflict, a flashing 

red text appears on the top of the datablock (such text 

is called “line 0” of a datablock), with “CA” indicating 

a conflict alert and “LA” indicating a low-altitude alert. 

Once the alert is acknowledged, controllers click the 

datablock to stop it from flashing. The alert text remains 

red until the conflict is resolved. Thus, red is first used to 

draw attention to a conflict. This purpose is also served 

by flashing as a redundant cue. There are two additional 

redundant cues to alert controllers of emergency situations on line 0. One is an audible alarm that sounds 

until the alert is acknowledged. The second is a conflict 

alert list that lists those targets in tabular form that are 

in conflict or low altitude alert. After a controller acknowledges the presence of a conflict by stopping the 

flashing, the alert message remains red for the controller 

to identify the aircraft that is in conflict, so red is also 

used for identification. 

Two colors are used for datablocks to indicate the 

ownership of aircraft: White datablocks are for owned 

aircraft, the ones for which the controller is responsible; 

green datablocks are for unowned aircraft, the ones that 

the controller does not control. Ownership is also indicated 

by a position symbol (a letter representing the sector that 

the controller is working with) attached to the datablock. 

Thus, white and green are used for identification, with 

text being the redundant cue. 

Yellow is used to identify datablocks of aircraft that 

are of interest to other controllers. The datablock of an 

aircraft owned by controller A is white on controller A’s 

display and depicts a full datablock. The datablock of 

the same aircraft is green on controller B’s display and 

is depicted by a partial datablock. When controller A 
4

needs to point out that aircraft to controller B, controller A can initiates a “pointout” action which means the 

datablock becomes a full datablock on B’s display, turns 

yellow in color, the letters PO appear at the end of line 

two, and the entire datablock flashes. When controller 

B acknowledges the pointout by slewing the cursor over 

the datablock and depressing the left acknowledge key, 

the datablock stops flashing, goes to a steady state, but 

remains yellow in color on his display. 

We first determined the background color and the 

default color of distracters. A default color is the color 

used for the majority of displayed materials in a display. 

Computer programmers use three numbers to specify a 

color: r for red, g for green, and b for blue. For example, 

the rgb values of red are (255, 0, 0) at its maximum luminance. Default colors are white (255, 255, 255) and 

green (0, 255, 255). 

Next we determined the purposes of color use, followed by the computation of the effect of color for CDs 

relative to that for CNs. If a color was used for multiple 

purposes, the analysis was performed for each purpose 

separately. For example, red was used for attention and 

identification. The effect of red was subsequently analyzed 

twice, once for each purpose. 

Red for attention

The purpose of using red is to draw a user’s attention to an alerted datablock among many other white 

and green datablocks. The red-colored “Line 0” text is 

accompanied by flashing, which is more effective than 

color for attention. Therefore, the effect of the redundant 

cue relative to the color is classified as “E.” For CDs, 

both the luminance difference and chromaticity difference between red and green are below the threshold for 

attention, so the effect of color for CDs is classified as 

“NE.” The overall effect of the red alert messages is “E,” 

because the flashing redundant cue is sufficient to serve 

the purpose even though the color use is not effective. As 

for text readability, even CNs cannot reliably read flashing 

text. Thus, the readability is classified as “NA.”

Red for identification

After a controller acknowledges the alert and stops 

the flashing, the alert text remains red for the controller to identify the conflicting datablocks from those in 

white, green, and yellow. The red text in “Line 0” of the 

datablock is either “CA” for collision alert or “LA” for 

low altitude alert. In addition, there is text in a conflict 

alert list that lists those targets in tabular form that are in 

conflict or low altitude alert. Thus, these two forms of text 

can serve as redundant cues for the color; however, they 

are less effective than color in identifying information. 

In addition, since the alert message “CA” or “LA” always 

appears in the position “Line 0” (which is on the top of 

a datablock), location serves as another redundant cue. 

Location is also less effective than color. For CDs, the 

chromaticity difference between red and green is below 

the threshold for identification; thus, red is not effective 

for differentiating datablocks from those in green. The 

overall effect of red alert messages in identification is 

categorized as “L” because of the effect of the redundant 

cues. The luminance contrast between the red text and 

dark background is below the reading threshold, even for 

CNs. Thus, the readability belongs to the “NE” category. 

Consistent with this finding, many controllers believe 

that red alert messages are difficult to read.

Yellow for identification

The purpose of yellow is to identify point-out 

datablocks from white and green ones. The redundant 

cues are flashing and the text “PO” before a controller 

acknowledges it and only the text PO remains after the 

acknowledgement. The effectiveness of the text redundant 

cue for identification is “L”. Since CDs cannot reliably 

name yellow from green, yellow is not effective for the task 

purpose. Therefore, the overall effect of yellow is classified as “L” in identifying datablocks for CDs. Since CDs 

perceive about the same luminance of yellow as CNs do, 

the yellow text on a dark background renders equivalent 

text readability for both CDs and CNs. 

Green for identification

Green and white are used to identify ownership of 

aircraft with which datablocks are associated. Green 

identifies the aircraft as unowned by the controller. 

A letter called the position symbol also indicates the 

ownership of an aircraft. The position symbol indicates 

the sector or position that has control of an aircraft. It is 

superimposed on the target and beacon symbol. A leader 

line extends from the target/beacon symbol/position 

symbol to the datablock. Thus, the position symbol can 

serve as a redundant cue, although it is less effective than 

color in complex scenes. For CDs, green is less effective 

to identify targets than white. The overall effect is “L” 

because both the redundant cue and the color for CDs 

are less effective. CDs can read the green text on a dark 

background as well as CNs do.

Weather on ACD

Filled background areas in the traffic situation display 

represent weather information. ACD has six weather 

levels: Levels 1-2 for moderate, Levels 3-4 for heavy, and 

Levels 5-6 for severe weather. It is crucial for controllers 

to distinguish different weather levels. Weather levels 

are represented by color and shades. Levels 1-2 are gray, 

Levels 3-4 are orange, and Levels 5-6 are orange-red. The 
5

areas with Levels 1, 3, and 5 weather are uniformly filled 

with the corresponding color. The areas with Levels 2, 

4, and 6 weather are filled with the corresponding color 

and white stipples. It is desirable for controllers to notice 

heavy weather immediately. Thus, orange and orange-red 

colors are used to draw attention and to identify the more 

severe types of weather.

Without color information, the only way controllers 

can identify weather levels is through the weather buttons 

in the menu bar on the top of an ACD display. Three 

menu buttons are toggle switches for weather Levels 1-2, 

3-4, and 5-6, respectively. If a controller could not identify 

a weather level by its color, he/she could turn on and off 

the weather buttons to determine the level. However, that 

is a tedious procedure. Controllers usually turn off Level 

1-2 weather and leave levels 3-4 and 5-6 on all the time. 

In this situation, although no redundant visual cues are 

associated with the color-coded objects, controllers can 

infer the information from the cues in other parts of the 

display. We refer to such cues as “Inference.”

Orange and orange-red for attention

The color orange is specified with the rgb values of 

(139, 90, 0). Orange-red is specified with (205, 55, 0). 

The shape of a filled weather area appears different from 

other materials such as datablocks, aircraft symbols, or 

boundaries. Thus, such a filled area may draw attention 

if it is significantly larger than the distracters; however, 

the shape cue is still less effective than color, so the effect 

of the redundant cue is categorized as “L” or “NE.” For 

CDs, orange and orange-red are not effective in drawing 

attention from white and green distracters because of their 

lower luminance and weak color difference from green. 

The overall effect is “L,” as determined by the effect of 

the redundant cue. 

Orange and orange-red for identification

Another purpose of these two colors is to identify 

Levels 3-4 and Levels 5-6 from Levels 1-2 (gray) weather. 

The redundant cue is the inference from the weatherlevel buttons in the menu bar. The effectiveness of such 

an inference cue is less than that of color. For CDs, the 

chromaticity differences between orange and orange-red, 

orange and gray, as well as orange-red and gray are all 

above the identification threshold but are less for CDs 

than CNs. Thus, the overall effect is classified as “L.” 

Since filled weather areas overlay the situation display, the text readability of datablocks is a concern for 

weather displays. While controllers should be able to read 

datablocks through filled weather areas, the readability 

of green datablocks on an orange or orange-red weather 

background is below the reading threshold. Fortunately, 

in real operations, aircraft rarely fly into heavy weather 

because a controller would re-route the aircraft. Thus, 

readability of datablocks in a weather area may not be a 

practical issue of concern. 

Results of color use for ACDs are summarized in Table 

1. The results for each color usage are listed in a row. 

Table 1. Analysis of color use in ACD 

Display 

Component 

Color Usage Purpose Redundant 

Cue 

Eff-R Eff-C Overall

Effect

Text

Readability 

Alert Red Text Att. Flashing E NE E NA 

Alert Red Text Id. Text / 

Location 

L NE L NA 

Pointout Yellow Text Id. Text L NE L E 

Datablock 

Unowned 

aircraft

Green Text Id. Text L L L E 

Level-3,4 Orange Filled area Att. Shape L / 

NE

NE L NA 

Level-3,4 Orange Filled area Id. Inference L L L NE 

Level-5,6 OrangeredFilled area Att. Shape L / 

NE

NE L NA 

Weather 

Level-5,6 OrangeredFilled area Id. Inference L L L NE 

Eff-R – Effect of redundant cues relative to colors. 

Eff-C – Effect of colors for CDs relative to CNs. 

Att. – attention. 

Id. – identification. 

L / NE – The effect could belong to either category depending on other attributes of the colored target such as the size or shape.

Table 1. Analysis of color use in ACD.
6

If a color is used for both attention and identification, 

the results for that color are listed in separate rows. The 

information presented in Table 1 columns, from left to 

right, includes the following:

•	 Display component: The functional component of a 

display. 

•	 Color: The color used in the component.

•	 Usage (of color): The visual stimuli that a color is applied to, such as text, box frame, symbol, filled area, 

or line.

•	 Purpose: The purpose of color use (attention or identification).

•	 Redundant cue: The achromatic visual attributes that 

are used along with the color.

•	 Eff-R: The effect of the redundant cue relative to the 

color.

•	 Eff-C: The effect of color for CDs relative to CNs.

•	 Overall effect: The overall effect by considering Eff-R 

and Eff-C together. 

•	 Text readability: The readability of the text associated 

with the color use.

Standard Terminal Automation Replacement System

STARS uses more colors than other primary ATC 

displays. Essentially every component in a STARS display 

involves color. The components where color use is taskcritical include system status alert, datablocks, weather, 

geographic restricted area, coordination list, and target 

symbol. We analyzed the use of color in these components, 

described the results, and summarized them in Table 2. 

System status alert 

The STARS display has a text box to indicate the system 

status. Within the box there are text fields for system failure 

alert, system overload alert, and radar failure alert. A text 

field is blank if there is no alert. If an alert is detected, 

red text appears in the corresponding field. Thus, red is 

used to draw the controller’s attention to the alert. The 

background of the text box is black. The default colors 

of other materials in the area are white and gray, which 

are the text colors in other menu boxes. 

Red for System Status Alert attention

Red is used to draw attention to alert messages. The 

redundant cue is the onset of the alert texts in a fixed location. The texts are not effective for attention. The onset 

of the texts may be able to draw attention, but the effect 

is less than that of color. For CDs, red is not effective to 

draw attention from white distracters. The overall effect 

is “L.” The luminance contrast of the red texts on a black 

background for CDs is below the reading threshold, so 

we classified the readability as “NE.”

Datablocks 

The default color for datablocks is green. A controller can highlight a specific datablock by clicking it to 

temporarily change the color to white. The alert messages for potential conflicts are similar to those on the 

ACD. Other than the red text of “CA” for conflict alert 

and “LA” for low altitude alert in “Line 0,” the alert 

messages of STARS also include several other types of 

alerts, all displayed in flashing red. Here, red is used for 

attention. Its effectiveness for CDs is “NE” due to the 

low luminance.

Aircraft symbol 

An aircraft symbol on STARS can be a Search Target 

Symbol (primary), or a Beacon Target Symbol (secondary), 

or both. The symbol consists of a small 2-dimentional 

graphic shape. The Search Target Symbol is displayed 

in green, and the Beacon Target Symbol is displayed in 

bright yellow. Thus, the purpose of yellow and green is 

to identify types of symbols. The chromaticity difference 

between green and yellow for CDs is below the identification threshold, thus the effect is “NE.” No visual redundant cue is used for the two types of symbols. However, 

controllers can infer the target type from the associated 

datablock. Therefore, the effect of the redundant cue 

is “L.” Taken together, the overall effect of color use in 

aircraft symbols is “L.” 

Weather

Weather information is presented on STARS with 

filled background areas that overlay the datablocks and 

other symbols. Because it must be possible to see aircraft 

datablocks, list data, radar targets, position symbols, 

history trails, and map data through the weather data, 

weather information on STARS is drawn with two lowluminance colors. Up to six levels of weather precipitation 

are displayed independently or simultaneously based 

on the users’ selection from the Display Control Bar. 

Weather levels are displayed as filled background areas 

using combinations of overlay stipples and color. Thus, 

controllers can determine each weather level in isolation 

based on the coding used. The coding schemes for weather 

levels are as follows: 

•	 Weather Level 1 - Dark gray-blue

•	 Weather Level 2 - Dark gray-blue, low-density bright 

gray overlay stipple

•	 Weather Level 3 - Dark gray-blue, high-density bright 

gray overlay stipple

•	 Weather Level 4 – Dark mustard 

•	 Weather Level 5 – Dark mustard, low-density bright 

gray overlay stipple

•	 Weather Level 6 – Dark mustard, high-density bright 

gray overlay stipple
7

Table 2. Analysis of color use in STARS. 

Display 

Component 

Color Usage Purpose Redundant

cue

Eff-R Eff-C Overall

Effect

Text

Readability

System Status 

Alert

Alert Red Alert Att. Location L NE L NE 

Datablock Alert Red Alert Att. Flashing E NE E NA 

Alert Red Alert Id. Text/ 

location 

L L L NE 

Primary 

target 

Green 2-D 

graphic 

symbol 

Aircraft Id. Inference L NE L NA 

Symbol 

Secondary 

target 

Yellow 2-D 

graphic 

symbol 

Id. Inference L NE L NA 

Levels1,2,3 Dark 

Grayblue 

Weather Filled area Id. Inference L L L E 

Levels4,5,6 Dark 

Mustard 

Filled area Id. Inference L L L E 

Message

status

Coordination Green Text Id. None NA NE NE E 

List

Message

status

Yellow Text Id. None NA NE NE E 

Restriction Area Beige Filled area Id. Shape E E E L 

Table 2. Analysis of color use in STARS.
8

The dark gray-blue is specified with the rgb values (57, 

115, 115) and dark mustard is specified with (124, 124, 

64). Since the three lower levels (Levels 1, 2, 3) use the 

same texture patterns as the three higher levels (Levels 

4, 5, 6), color is the only visual cue that distinguishes 

between lower and higher levels. Without color, a controller still can identify the weather level by selectively 

turning on and off the weather-level selection boxes in 

the Display Control Bar. Nevertheless, the procedure is 

tedious; thus, the effectiveness is less than what would 

be produced by color. 

Dark gray-blue and dark mustard for identification

The purpose of the colors is to identify weather levels. 

The redundant cue is the inference from weather-level 

selection boxes in the menu bar. The chromaticity difference between dark gray-blue and dark mustard for CDs 

is above the identification threshold but is less than the 

difference for CNs. Thus, the overall effect is classified 

as “L.” The text readability of green datablocks on a 

gray-blue or mustard background is about the same for 

CDs as for CNs.

Coordination list 

The coordination list provides information about the 

coordination status between controllers. Three colors 

are used to identify the status of text messages: white for 

unsent messages, green for acknowledged messages, and 

yellow for non-acknowledged messages. No redundant 

cues are used. CDs perceive white and yellow in roughly 

the same way as CNs do. However, CDs perceive green 

as dark yellowish. The chromaticity difference between 

yellow and green for CDs is below the identification 

threshold. Thus, the two colors are not effective for 

identification. The text readability is about the same for 

CDs as that for CNs.

Geographic restriction area 

A geographic restriction area is indicated with a circle 

or polygon filled with beige color, specified by rgb values 

of (225, 150, 90). The text displayed within the circle 

or polygon is green. Although beige is used to identify 

geographic restriction areas, the shapes of such areas are 

distinctive from other materials in the display and can be 

reliably named (circle or polygon), so they are as effective 

as color for the identification purpose. Thus, we classify 

the effect of the redundant cue as “E.” The readability 

of the green text on a beige background is less for CDs 

than that for CNs.

Display System Replacement 

Presently, DSR does not use many colors. The background color of a DSR can be adjusted between dark blue 

and black. Datablocks are green. Conflict alerts are indicated by flashing texts; no color cues are utilized. Currently, 

only two components of a DSR display use color-coding: 

the graphic tool and the Weather And Radar Processor 

(WARP). The use of color in these two components is 

described below and summarized in Table 3.

Graphic tool

The graphic tool allows controllers to select one of the 

four given colors to draw filled areas for specific flying 

regions. The four colors are red, yellow, green, and white. 

The distance of a chosen aircraft to a specified region is 

subsequently displayed with the same color. Thus, the 

colors are used to identify information. For CDs, the 

chromaticity differences between red and green, red and 

yellow, as well as green and yellow are all below the identification threshold. Therefore, red, yellow, and green are 

not effective. In addition, the readability of red text on a 

dark blue background is below the reading threshold. 

Weather And Radar Processor

WARP displays specialized aviation weather products to 

support en route air traffic control operations. It provides 

Next Generation Doppler Weather Radar (NEXRAD) 

data for DSR displays. NEXRAD data are classified into 

six weather levels. Levels 1-2 are moderate weather, and 

levels 3-6 are heavy weather that requires controllers’ imTable 3. Analysis of color use in DSR. 

Display Component Color Usage Purpose Redundant 

Cue 

Eff-R Eff-C Overall 

Effect

Text

Readability 

Red Filled area/ 

Text

Id. NA NE NE NE NE 

Yellow Filled area/ 

Text

Id. NA NE NE NE E 

Graphic Tool 

Green Filled area/ 

Text

Id. NA NE NE NE E 

Weather Levels 1,2 Purple Filled area Id. Text L NE L NE 

Levels 5,6 Turquoise Filled area Id. Text L NE L NE 
9

mediate attention. Two colors and one colored pattern 

are used to display NEXRAD data in filled background 

areas on the DSR: purple for Levels 1-2, turquoise and 

black checkerboard for Levels 3-4, and turquoise for 

Levels 5-6. The number of each level is displayed in white 

overlaying on the filled weather areas. 

Turquoise and purple for weather identification

The purpose of these two colors is to identify weather 

levels. The white text is a redundant cue indicating the 

weather level. The text is less effective than colors in 

identification. The chromaticity difference between turquoise and purple is below the identification threshold 

even for CNs. Thus, the effect of the colors is classified 

as “NA.” The overall effect is “L.” The readability of 

the white text on turquoise and purple backgrounds for 

CDs is roughly the same as that for CNs. However, the 

readability of green datablocks on a turquoise or purple 

background is below the reading threshold. 

User Request Evaluation Tool

URET has three display windows that are frequently 

used by controllers: Aircraft List, Graphic Display, and 

Plans Display. The background color for each window 

is black. The default color is white. Next, we described 

the use of color in the three windows and summarized 

the results in Table 4.

Aircraft List 

The Aircraft List displays several types of text information, including flight status, flight ID, altitude, and 

routes. Flight status is indicated with text boxes in the three 

left-most columns. The frames of the boxes and the text 

within the box are color-coded by red / muted red, yellow 

/ muted yellow, cyan, and brown. In the first column, the 

boxes may be colored red or muted red. Red represents 

predicted loss of separation within 5 minutes. Muted red 

represents predicted loss of separation if the controller does 

not take other expected actions first. In other words, unless 

the controller takes some action, a loss of separation will 

develop. Therefore, both red and muted red serve as alerts 

that require controllers’ immediate attention. The text box 

for a flight in the normal status is shown as a white box 

frame without text in it. When the loss of separation is 

predicted, the box frame turns red or muted red, and a 

letter “R” appears in the box in the same color as the box 

frame. In the second column, yellow and muted yellow 

represent potential conflicts between aircraft predicted to 

come within 5-12 minutes of each other. Thus, yellow 

also requires attention. In addition, each of these colors 

has a different meaning, and controllers must associate 

the color with its meaning. Therefore, we classified the 

purposes of these colors as attention and identification. 

Cyan in the third column of boxes indicates aircraft that 

are predicted to conflict with a restricted airspace. Brown 

may appear in all three columns for a single flight. Brown 

means that auto-prediction may not be available because 

no tracking data for the flight is available for URET. A 

brown box frame is accompanied with a symbol “X” in 

it, so the symbol serves as a redundant cue. 

The column for flight ID uses white text for normal 

flights, brown for the flights without tracking data, and 

orange for overdue flights. No redundant cue is used. 

The altitude column uses white texts for filed altitude 

and yellow texts for inappropriate altitude. Again no 

redundant cue is used. Route information is displayed 

with white and cyan texts. Those colors are used for 

segmentation; thus, they are not included in the analysis 

below. In fact, controllers rarely rely on colors for route 

information because they must read the text to acquire 

information.

Red and muted red for attention

The redundant cue is the onset of the letter within 

the boxes. Because the letter is very small, its onset is 

not conspicuous enough to capture attention. Red is not 

effective to draw attention for CDs. Thus, both red and 

muted red are not effective for CDs.

Red and muted red for identification

While red and muted red are used to identify different 

types of flight status, they only differ in their luminance. 

The difference between them is not significant enough for 

CDs to reliably identify information. No redundant cue 

is used to tell red from muted red. Thus, the overall effect 

of color use is “NE.” In addition, the text readability is 

below the reading threshold for CDs and CNs.

Yellow and muted yellow for attention

The colors are used to draw attention to flights that 

are predicted to come within 5-12 miles of each other. 

The redundant cue is a single letter “Y,” which is not 

conspicuous enough to capture attention. Yellow is less 

effective to draw attention from white distracters for CDs 

than for CNs. The luminance difference between muted 

yellow and a black background is below the attention 

threshold even for CDs and CNs. 

Yellow and muted yellow for identification 

The analysis is the same as that for red and muted 

red in that the colors only differ in their luminance. The 

difference between them is not significant enough for 

CDs to reliably identify information. Thus, the overall 

effect of color use is “NE.” No redundant cue is used. 

The readability of the yellow text on a black background 

for CDs is equal to that for CNs. 
10

Table 4. Analysis of color use in URET. 

Display Component Color Usage Purpose Redundant 

Cue 

Eff-R Eff-C Overall 

Effect

Text

Readability 

Aircraft 

List

Flight 

status

Red Text / 

Box 

frame 

Att. Text / 

Location 

NE NE NE NE 

Flight 

status

Muted 

red

Text /

Box 

frame 

Att. Text / 

Location 

NE NE NE NE 

Flight 

status

Yellow Text / 

Box 

frame 

Att. Text / 

Location 

NE L L E 

Flight 

status

Muted 

yellow 

Text /

Box 

frame 

Att. Text / 

Location 

NE L L E 

Flight 

status

Red Text / 

Box 

frame 

Id. None NA NE NE NE 

Flight 

status

Muted 

red

Text,

Box 

frame 

Id. None NA NE NE NE 

Flight 

status

Yellow Text / 

Box 

frame 

Id. None NA NE NE E 

Flight 

status

Muted 

yellow 

Text /

Box 

frame 

Id. None NA NE NE E 

Flight 

status

Cyan Text / 

Box 

frame 

Id. Text / 

Location 

L L L E 

Flight 

status

Brown Text / 

Box 

frame 

Id. Text L L L NE 

Flight ID Orange Text Id. Text L E E E 

Altitude Yellow Text Id. None NA L L E 

Datablock 

/ route 

Red Text / 

Line

Id. None NA NE NE NE 

Datablock 

/ route 

Brown Text / 

Line

Id. None NA NE NE NE 

Graphic 

Display 

Datablock 

/ route 

Yellow Text / 

Line

Id. None NA NE NE E 

Plan

evaluation

Plan Red Text Id. Text L NE L NE 

Display 

Plan

evaluation

Green Text Id. Text L NE L E 
11

Cyan for identification

Cyan is used to distinguish aircraft routes from those 

in white. The chromaticity difference between cyan and 

white is slightly less for CDs than for CNs. The redundant 

cue is the presence or absence of a letter “C” inside the 

text box. The cue is less effective than color. Therefore, 

the overall effect of cyan for CDs is “L.” 

Brown for identification

The purpose is to distinguish aircraft from those displayed 

in red, yellow, cyan, and white. The redundant cue is the 

presence or absence of a letter “X” in the text box. The cue 

is less effective than color. The chromaticity differences 

between brown and red, and brown and yellow for CDs 

are above the identification threshold but less than those 

for CNs. Thus, the overall effect is “L.” The readability of 

the letter “X” for CDs is below the reading threshold.

Yellow for inappropriate altitude identification

The purpose is to highlight an aircraft’s altitude from 

those shown in white. No redundant cue is used. The 

chromaticity difference between yellow and white for 

CDs is slightly less than that for CNs. The readability of 

the yellow text on a black background is about the same 

for CDs as that for CNs.

Graphic Display

The Graphic Display illustrates datablocks and routes 

for the aircraft in a sector. White datablocks and routes 

indicate normal flights. Red, muted red, yellow, muted 

yellow, brown, and cyan colors have the same meanings 

as those used in the Aircraft List. No redundant cues are 

used for the colors. 

The purpose of the color use is to identify the conflict 

status of different types of aircraft and routes when planning a route change. For CDs, the chromaticity differences 

between red and brown, red and yellow, and yellow and 

brown are all less than the identification threshold. Thus, 

these colors are not effective for CDs. No redundant cue 

is used. The overall effect is “NE.” In addition, the readability of red and brown datablocks for CDs is less than 

the reading threshold. The readability of yellow datablocks 

for CDs is equivalent to that for CNs. 

Plans Display

The Plans Display shows text information about the 

evaluation of a proposed flight plan change. Green is used 

for no-confliction plans, and red indicates a conflict. The 

purpose of color use is for identification. The text itself is 

a redundant cue. However, it is less effective than colors. 

The chromaticity difference between red and green for 

CDs is below the identification threshold. Thus, the 

overall effect of the colors is “L.” The readability of the 

red text for CDs is less than the reading threshold; the 

readability of the green text is above the threshold and 

equal for CDs and CNs. 

Traffic Management Advisor

TMA provides a number of graphical features to improve controllers’ situational awareness. The frequently 

used graphical features include Timeline and Load Graphs. 

Both use color-coding. The use of color in these two 

graphical products is described next, and the results are 

summarized in Table 5.

Table 5. Analysis of color use in TMA. 

Display Component Color Usage Purpose Redundant 

Cue 

Eff-R Eff-C Overall 

Effect

Text

Readability 

Aircraft tag Yellow Text Id. None NA NE NE E 

Aircraft tag Green Text Id. None NA NE NE E 

Aircraft tag Turquoise Text Id. None NA L L E 

Delay time Green Text Id. Text L NE L E 

Delay time Yellow Text Id. Text L NE L E 

Delay time Orange Text Id. Text L NE L E 

Delay time Red Text Id. Text L NE L NE 

Delay time Red Text Att. Text NE NE NE NE 

Timeline 

Rush alert Red Graphic 

symbol 

Att. Shape L NE L NA 

Traffic

volume 

Load Yellow Curve Id. None NA NE NE E 

Graph 

Traffic

volume 

Green Curve Id. None NA NE NE E 
12

Timeline

A timeline displays text information about time scales, 

aircraft tag, delay time, and the reference point along 

a double-lined vertical axis. The background color is 

black. The default colors are green for the majority of 

texts and white for the axis (lines and ticks). An aircraft 

tag (i.e., the code of the airline company and the flight 

number) may appear in one of three colors: green for 

normal arrivals, yellow for unfrozen Scheduled Time of 

Arrival (STA), and turquoise for frozen STAs. A frozen 

STA means that the aircraft is close enough to the meter 

fix that TMA schedulers will not change its STA except 

in response to extraordinary events. An unfrozen STA 

means that TMA schedulers can change the displayed 

STA. An orange symbol next to an aircraft tag indicates 

that TMA has not received any radar tracks for that aircraft so that it computes time of arrival using the filed 

flight plan of the aircraft. 

The delay time for every aircraft is displayed as the 

number of minutes along the timeline. The numbers are 

also color-coded: green for delays less than 5 min, yellow 

for delays between 5 to 10 min, orange for delays between 

11 to 15 min, and red for delays longer than 15 min. 

Thus, colors are used to identify different types of delays, 

yet red is also used to capture TMA users’ attention. The 

Timeline also uses a pair of red brackets to show a rush 

time alert. The shape of the brackets is a redundant cue 

for the color. 

Yellow, green, and turquoise for aircraft tag identification

The purpose of these colors is to identify different 

types of aircraft arrivals. No redundant cue is used. The 

chromaticity difference between yellow and green for 

CDs is below the identification threshold. The difference between yellow and turquoise is less effective for 

CDs than for CNs. The difference between green and 

turquoise for CDs is as effective as that for CNs. The 

text readability of these colors for CDs is equivalent to 

that for CNs. 

Green, yellow, orange, and red for delay time 

identification

The purpose of these four colors is to identify four categories of delay time. The redundant cue is the text for the 

length of delay in minutes, and it is less effective than color. 

The chromaticity differences between green and yellow, 

green and red, as well as yellow and red for CDs, are all 

below the identification threshold. The differences between 

red and orange, as well as green and orange for CDs, are 

less effective than those for CNs. The readability of the 

red text is below the reading threshold, and the readability 

of the orange text is less for CDs than for CNs. 

Red for attention

Red is used to draw users’ attention to aircraft that have 

a delay time of longer than 15 minutes. It is not effective 

in drawing the attention of CDs. The redundant cue is 

the text. It is also not effective for the purpose. Thus, the 

overall effect of color use is “NE” for CDs. Timeline also 

uses a pair of red brackets to show a rush-time alert. The 

shape of the brackets is a redundant cue. It could draw 

some attention because the shape is different from other 

materials in the Timeline display. Nevertheless, the cue 

is less effective than color. 

Load Graph

The Load Graph shows present and future traffic flows 

over time. It displays two curves: a green one for the expected traffic demand and a yellow one for planned arrivals. 

No redundant cue is used. In addition, a red straight line 

shows the airport acceptance rate. This line is static over 

the time. The purpose of red is neither for attention nor 

for identification. It is simply to segment the line from 

the curves because they may overlay each other. 

Yellow and green for identification

The purpose of yellow and green in the Load Graph 

is to identify different types of traffic flows. Color is the 

only visual cue to distinguish the two curves, although 

users might be able to identify the curves based on their 

experience. The chromaticity difference between green 

and yellow for CDs is below the identification threshold, 

so the colors are not effective for them. 

Integrated Terminal Weather System

ITWS is an automated weather information system 

used in TRACON and tower facilities. ITWS integrates 

many weather products that detect and predict windshear, 

microburst, gust fronts, hail, lightning, and tornadoes. The 

system also displays precipitation, tracks the speed and 

direction of storm cells, and predicts storm movements. 

The display components of ITWS include Graphics 

Product Display, Product Status Buttons, Alert Panel, 

and Site Information Display. We analyzed the use of 

color in each component and summarized the results 

in Table 6.

Graphics Product Display

The background color of the Graphics Product Display is gray. This product displays six levels of weather 

precipitation as filled areas. The intensities of precipitation are based upon National Weather Service intensity 

levels. Levels 1 to 6 are indicated by light green, green, 

yellow, light orange, orange, and red, respectively. There 

is no redundant cue for the colors. The Graphics Product 

Display also displays additional weather types such as 
13

Table 6. Analysis of color use in ITWS.

Display 

Component 

Color Usage Purpose Redundant 

Cue 

EffREffC

Overall 

Effect

Text

Readability 

Preciptation Light 

Green 

Filled 

area

Id. None NA NE NE NA 

Preciptation Green Filled 

area

Id. None NA NE NE NA 

Preciptation Yellow Filled 

area

Id. None NA NE NE NA 

Preciptation Light 

Orange 

Filled 

area

Id. None NA NE NE NA 

Preciptation Orange Filled 

area

Id. None NA NE NE NA 

Preciptation Red Filled 

area

Id. None NA NE NE NA 

Preciptation Red Filled 

area

Att. None NA NE NE NA 

Preciptation Orange Filled 

area

Att. None NA L L NA 

Windshear 

Microburst 

Red Circle

/ Filled 

circle

Att. Shape L / 

NE

NE NE NA 

GuestFront Purple Solid / 

Dash 

line 

Att. Shape L / 

NE

NE NE NA 

Windshear 

Microburst 

Red Circle

/ Filled 

Circle

Id. Shape E NE E NA 

Graphics 

Product 

Display 

GuestFront Purple Solid / 

Dash 

line 

Id. Shape E NE E NA 

Red Filled 

Box 

Id. None NA NE NE NE 

Green Filled 

Box 

Id. None NA NE NE E 

Product 

Status

Buttons 

Yellow Filled 

Box 

Id. None NA NE NE E 

Windshear 

Microburst 

Red Filled 

Box 

Att. Luminance E NE E NE 

GuestFront Purple Filled 

Box 

Att. Luminance E NE E E 

Alert

Panel

Lightning Yellow Filled 

Box 

Att. Luminance E NE E E 

Airport code Red Filled 

Box 

Id. Inference L NE L E 

Airport code Yellow Filled 

Box 

Id. Inference L NE L E 

Site

Information 

Display 

Airport code Blue Filled 

Box 

Id. Inference L E E E 
14

microburst, wind shears, gust fronts, and tornado alerts 

overlaying areas of precipitation. Each of these weather 

types is indicated with a different color and a shape 

cue. Gust Fronts are displayed as solid purple lines corresponding to the location of detected gust fronts, and 

dashed purple lines indicate the predicted positions of 

the gust fronts in 10 and 20 min. A detected windshear 

is presented as an open red circle with the corresponding strength indicated inside the circle. A microburst is 

presented as a solid red circle with its strength indicated 

inside the circle. Detected tornadoes are presented as 

solid black triangles inside an open black circle in the 

area of detection. 

Light green, green, yellow, light orange, orange, and red 

for precipitation identification

These filled areas vary in color from green to red to 

identify increasing levels of precipitation. No redundant 

cue is used. The hue order of these colors for CDs is 

different from that for CNs, so CDs may not correctly 

interpret precipitation levels. The chromaticity differences between many pairs of these colors are below the 

identification threshold. Thus, the overall effect for these 

colors is “NE.”

Orange and red for precipitation attention

These two colors represent the heaviest precipitation 

levels and are intended to capture users’ attention. No 

redundant cue is used. Red is not effective for CDs, and 

orange is less effective for CDs than it is for CNs.

Red and purple for severe weather attention

These colors are used to capture users’ immediate attention. The distracters are the filled areas of precipitation 

levels; thus, the default color can vary from light green to 

red, depending on weather situations. In circumstances 

of bad weather, the precipitation areas are a mixture of 

green, orange, yellow, and possibly some red. Purple and 

red are not effective for CDs in drawing attention from 

such a mixture of colors. The redundant cues are the 

shapes (open circle, filled circle, and solid and dashed 

lines). The shapes usually cannot draw attention unless 

their size is significantly larger than that of distracters. 

Thus, we classified the effect as “NE.”

Red and purple for severe weather identification

These colors are also used to identify severe weather. 

However, color is not an effective cue for identification 

in this display, because some colors do not uniquely correspond to one type of weather. For example, red is used 

for both microburst and windshear; and purple is used 

for the current gustfront and the gustfront predicted in 

10 to 20 min. Due to the uncertainty in color-coding, 

most users naturally use shapes as the primary cue for 

identification. Thus, the effect of color in identification for CDs can be equivalent to that for CNs. The 

problem for CDs is that the visibility of those colored 

shapes becomes very low due to the shapes overlaying 

with color-filled precipitation areas. For example, a red 

circle for a windshear may not be detected by CDs if it 

overlays a green or red precipitation area. 

Product Status Buttons

Product Status consists of two rows of buttons. Each 

button has texts in black, indicating the meaning of the 

product represented by the button. For example, “Precip” 

represents precipitation, and “MB” represents microburst. 

The color of a button represents the status of the associated weather product in Graphics Product. If a product 

is unavailable in the Graphics Product Display, the button turns red. If a product is available and is turned on 

in the Graphics Product Display, the button is green. If 

the product is available and is being filtered, the button 

is yellow. If a product is available but is turned off, the 

button is white. If the product is not supported, the 

button is gray. Therefore, red, green, yellow, white, and 

gray are used to identify the status of weather products. 

There is no redundant cue for these colors. 

Red, green, and yellow for product status identification

The purpose of these colors is to identify the status 

of a weather product. No redundant cue is used for 

identification. Color differences between red and green, 

red and yellow, as well as green and yellow for CDs are 

all below the identification threshold. Thus, these colors 

are not effective for CDs to identify the product status. 

In addition, the readability of the black texts on a red 

background for CDs is below the reading threshold.

Alert Panel

The Alert Panel is located on the upper right-hand 

portion of the display. In the case of potentially hazardous 

weather detected in a terminal area, six boxes in this panel 

may light up to alert users to specific conditions. The six 

alert products are: Microburst Alert (red), Windshear Alert 

(white), Gust Front within 20 minutes (purple), Lightning 

Alert within 20 miles (yellow), Tornado Alert within 

10 miles (black), and Anomalous Propagation (black). 

The background of the boxes is gray when the weather 

is not available. By lighting these boxes with colors, the 

display draws the attention of users to a situation that 

is potentially dangerous, or may require a change in the 

runway configuration or airspace. Therefore, the colors 

are used mainly for attention. The redundant cue is the 
15

sudden onset of colors at fixed locations. The colors also 

help users to identify types of weather, with location and 

text being the redundant cues. The default text color is 

black for most of the boxes but white for the purple and 

black boxes to render good text readability. An exception 

is that the text is red to draw attention for the white box 

when a windshear alert is detected. 

Red, purple, and yellow for attention

Because the boxes in the Alert Panel form a heterogeneous field of multiple colors, these colors are not very 

effective in drawing attention. The redundant cue for 

attention is luminance. When the onset of a visual object 

draws attention, luminance plays a greater role than chromaticity (Yantis & Jonides, 1996). Therefore, the onset 

of luminance in the boxes is equally effective for CDs 

as for CNs, because CDs and CNs perceive luminance 

about the same. The readability of the black text on a 

red background is “NE” for CDs. The text readability of 

other text and background color combinations for CDs 

is equivalent to that for CNs. 

Site Information Display 

Site Information Display presents a table of airport 

weather situations. Airport codes are listed in the leftmost 

column. Other columns list operational mode, alert, and 

storm information. Each row is for one airport. The text 

background of airport codes is color-coded to represent 

the operational status of weather sensors: yellow for 

storm cells, red for action alert, blue for operational, 

and black for not-operational. The text of airport codes 

is white on a red, blue, or black background, and black 

on a yellow background. Without color, users are able 

to infer the status of airport weather by reading the text 

in other columns. Thus, the colors have an inference 

redundant cue. 

Red, yellow, and blue for identification

These colors are used to identify the weather status 

of airports, with texts in other columns serving as the 

inference redundant cue. Theoretically, the redundant 

text is less effective than colors. However, since the 

whole ITWS system uses many sets of color codes for 

identification, the effectiveness of colors in identification 

is greatly reduced in general. In real operations, users 

tend not to use color as the primary cue when there are 

too many colors in a display. Therefore, the effectiveness 

of color could be the same for CDs and CNs, because 

color is no longer a primary cue to acquire information. 

However, the chromaticity difference between red and 

yellow for CDs is below the identification threshold, so 

red and yellow are “NE.” The text readability for CDs 

is equivalent to that for CNs.

Statistics

In this section, we present the results in Tables 1-6 

graphically to provide an overall description of the effect 

of CDs in using ATC displays. 

Figure 1 shows the overall effect of color use across 

the three primary displays: ACD, STARS, and DSR. 

The left panel shows the effectiveness of color for attention and identification tasks. The right panel shows 

0

20

40

60

Percentage of color use

E L NE

Overall effect of color use

E L NE

Text readability

Figure 1. The statistic effect of color use for CDs relative to CNs in the three 

primary ATC displays: DSR, ACD, and STARS. The left panel shows the overall 

effect of color use for CDs relative to CNs in attention and identification tasks. 

The horizontal axis represents the three categories of color-use effectiveness: “E” 

for equivalent effective in CDs and CNs, “L” for less effective in CDs, and “NE” for 

not effective in CDs. The vertical axis represents the percentage of color usages 

in each category, calculated as the number of color usages in a given category 

divided by the total number of color usages for attention and identification. The 

right panel shows the effect of color use on text reading. The horizontal axis 

indicates the three categories of text readability for CDs relative to CNs: E, L, and 

NE. The vertical axis is the percentage of colored text in each category.
16

text readability associated with color use. In each panel, 

three categories of effectiveness, “E,” “L,” and “NE” are 

listed along the horizontal axis. The vertical axis is the 

percentage of occurrence in each category, calculated as 

the number of occurrences in a given category divided by 

the total number of times that color is used for attention 

and identification or text reading. 

Notice that about 10% of color usages belong to the 

“E” category, meaning that CDs can use color-coded 

information as effectively as CNs in those applications. 

More intriguingly, about 22% of color uses belong to 

“NE,” indicating that in these situations CDs might not 

be able to effectively use color-coded information in these 

ATC displays. Since the primary displays are critical for 

controlling traffic, this percentage may be high enough to 

represent a potential safety concern. Further task analysis and experimental validation needs to be performed 

regarding the 22% in the NE category for color usages 

to elucidate the operational effects of color use on ATC 

task performance. The right panel shows that about 30% 

of color usages in text result in less-than-threshold text 

readability, suggesting that CDs may encounter some 

difficulty in reading text associated with color use.

Figure 2 is in the same format as Figure 1, showing the 

data for the three supporting displays: URET, TMA, and 

ITWS. Compared with the results in Figure 1, about 50% 

of color usages for attention and identification belong in 

the “NE” category. The right panel of Figure 2 indicates 

that 35% of color usages in text could result in text reading 

problems. These cases also require further investigation. 

Figure 3 shows the effect of redundant cues across 

the three primary displays, DSR, ACD, and STARS. 

The horizontal axis is comprised of the four categories 

of effectiveness of redundant cues relative to color. The 

category “NA” denotes color usages where no redundant 

cues are applied in addition to color. There are several 

cases in which the effect is uncertain, depending on individual situations. For example, if a filled orange-red area 

for heavy weather were significantly larger than the area 

of a datablock, then shape would be as effective as color 

in capturing attention. However, if the area is less than 

that of a datablock, then its capacity to attract attention 

is limited. In such cases, we used the lowest possible scale 

for the graph, e.g., the effect is the orange-red in attention 

is “L” as Figure 3 is concerned. 

The results in Figure 3 are intriguing. While most color 

use guidelines emphasize the importance of redundant 

cues, about 20% of color usages for attention and identification tasks belong to the “NA” category, meaning that 

colors were used without redundant cues. Moreover, even 

when redundant cues are used, in most cases the effect of 

the cue is either less than the effect of color, or it is not 

effective for the given task purposes. Among all the color 

usages for attention and identification, only about 8% 

have redundant cues that are as effective as colors.

Figure 4 shows the effect of redundant cues in the three 

supporting displays, TMA, URET, and ITWS. The figure 

is in the same format as that of Figure 3. The percentages 

of the “E” and “NE” categories are similar to those in the 

primary displays. However, the percentage of the “NA” 

category is much higher – 46% of color usages for attention and identification in the supporting displays do not 

have redundant cues. This high percentage would seem 

to imply that the human factors effort in the interface 

designs of supporting displays was inadequate. 

CONCLUSIONS AND DISCUSSION 

In this report, we analyzed the effect of color use in 

ATC displays on CDs. The analysis was performed for 

three primary displays: DSR, ACD, and STARS, and three 

supporting displays: URET, TMA, and ITWS. For each 

display, we first identified the situations where color was 

used as a primary cue for attention or identification tasks. 

For those situations, we identified achromatic redundant 

cues (if any) and assessed their effectiveness relative to 

colors. We then applied the algorithms developed in our 

previous report (Xing & Shcroeder, 2005b) to compute 

the effectiveness of color for CDs relative to CNs. Finally, 

if color was used in texts, we computed and compared 

the text readability for CNs and CDs. These three values 

gave an overall description of how color vision deficiencies 

might affect the use of color displays.

The Effect of Redundant Cues

We often hear the argument that CDs can use redundant cues to perform color-related tasks. However, our 

analysis showed that redundant cues should be chosen 

carefully to fit the task purpose. Otherwise, the cues could 

be either less effective than colors or not effective at all. 

For example, if a controller needs to devote immediate 

attention to a target, then the redundant cue should be 

flashing or having a luminance much greater than other 

materials displayed in the area. Text and shapes are not 

effective for this purpose. Also, if a controller needs to 

identify different types of information, then the redundant 

cues should be visually distinctive from each other and 

should be reliably named. A good example of redundant 

cues for identification is the use of distinctive shapes in 

ITWS, such as lines and circles to identify severe weather 

in addition to colors. An opposite example is using text 

as a redundant cue for identification. Visual experiments 

have demonstrated that text is less effective than colors 

for identification in complex scenes (Christ, 1975). 
17

0

20

40

60

Percentage of color use

E L NE

Overall effect of color use

E L NE

Text readability

Figure 2. The statistic effect of color use for CDs relative to CNs in the three 

supporting ATC displays (URET, TMA, and ITWS). The format of the figure is the 

same as that of Fig. 1.

Figure 4. The effect of redundant cues relative to colors 

for color usages in the three supporting displays (URET, 

TMA, and ITWS).

0

0.1

0.2

0.3

0.4

0.5

0.6

Percentage of color use

E L NE NA

Effect of redundant cues

0

0.1

0.2

0.3

0.4

0.5

0.6

Percentage of color use

E L NE NA

Effect of redundant cues

Figure 3. The effect of redundant 

cues relative to colors for color 

usages in the three primary 

ATC displays (ACD, STARS, 

and DSR). The analysis was 

performed only on the color 

usages with a task purpose of 

attention or identification. The 

horizontal axis indicates the 

categories of the effectiveness of 

redundant cues relative to colors, 

with “E” for equivalent, “L” for 

less effective, and “NE” for not 

effective for CDs. In addition, the 

“NA” category is denoted to the 

situations where color is used for 

attention or identification without 

redundant cues.
18

Text Readability

Through the analysis, we noticed that there were many 

cases where text readability could be a potential problem 

for CDs. In fact, we reported only a portion of the cases 

used in ATC displays. We calculated the text readability 

for those situations where color was used for attention or 

identification. In ATC displays, color is also widely used 

to segment texts from other materials. Moreover, color 

is often applied to texts without an apparent underlying 

rationale. In those situations, while color should not 

affect CDs’ task performance because it does not have 

any meaning, it can introduce low text readability. Given 

that a large portion of ATC tasks involves text reading, 

any new color vision standard should take this factor 

into consideration. 

Limitations of the Analysis

Finally, we need to point out that the analysis in this 

report only provides a baseline assessment of the effect of 

color vision deficiencies. One limitation in our analysis is 

the effects of color vision deficiencies. The algorithms we 

used are based on those developed by Brettel et al. (1997). 

It simulates severe color vision deficiencies, in which one 

or two types of cones are completely absent from retinas. 

Many individuals have mild color vision deficiencies, in 

which the number of a certain type of cones is less than 

normal, or the cones are less sensitive than normal. So, 

the effects we estimated may differ, depending on the 

extent of the color vision deficiency. Another limitation 

is the lack of operational considerations. Since there is a 

high redundancy in aviation, controllers may use other 

sources to acquire color-coded information. Therefore, 

while there are a number of cases in Tables 1-6 where 

color use has the potential to negatively affect task performance of CDs, in real operations color may or may 

not cause problems for CD controllers, depending on 

individual situations and other redundancies in the ATC 

system. The purpose of this report is to narrow the range 

of situations where color use has the potential to cause 

problems for color-deficient controllers. A further step is 

to conduct a task analysis to determine how critical it is 

for controllers to be able to use color-coded information, 

and determine whether controllers have other ways to get 

around color vision deficiencies. 

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