Color model

A color model is a mathematical model describing the way to represent colors as numbers, usually as three or four values. These values are called color components or color channels. When such model is linked to a description of how to interpret the components (viewing conditions, etc.), the resulting set of colors is called "color space."
Additive and subtractive color models
The color models can be classified as additive and subtractive.
In additive color models, such as RGB, white is the combination of all primary-colored lights, and black is the absence of any light. Other colors are a mix of different strength of primary colors.
Subtractive color models subtract or mask colors from white background of the paper. That is the ink reduces the reflected white light. White light minus red leaves cyan, white light minus green leaves magenta, and white light minus blue leaves yellow. Such color models are usually used in printing. An example of such a model is CMYK color model.
Additive and subtractive color models are opposite.
RGB color model
Media that emit light (such as television) use color mixing with primary colors of red, green, and blue, each of which stimulates one of the three types of the eye's color receptors. This is called "RGB" color model. Mixtures of light of these primary colors cover a large part of the human color space.
RYB color model
RYB (an abbreviation of red-yellow-blue) is an additive color model that uses red, yellow, and blue pigments as primary colors in art and applied design. The RYB color model underpinned the color curriculum of the Bauhaus.
CMY and CMYK color models
It is possible to achieve a large range of colors seen by humans by putting cyan, magenta, and yellow transparent dyes/inks on a white background. These are the subtractive primary colors. Often a fourth ink, black, is added to improve reproduction of some dark colors. This is called the "CMY" or "CMYK" color model.
The cyan ink absorbs red light but transmits green and blue, the magenta ink absorbs green light but transmits red and blue, and the yellow ink absorbs blue light but transmits red and green. The white substrate reflects the transmitted light back to the viewer.
Cylindrical-coordinate color models
A number of color models exist in which colors are fit into conic, cylindrical or spherical shapes, with neutral (gray) colors running from black to white along a central axis, and hues (tints) corresponding to angles around the perimeter. Models of this type date back to the 18th century, and continue to be developed in the most modern models.
Different color theorists have each designed unique color 3D shapes. Many are in the shape of a sphere. The color spheres invented by Phillip Otto Runge and Johannes Itten are typical examples and prototypes for many other color solid schematics.
HSL and HSV
HSL and HSV are both cylindrical, with hue, their angular dimension, starting at the red primary color at 0deg, passing through the green primary at 120deg and the blue primary at 240deg, and then going back to red at 360deg. In each, the central vertical axis comprises the neutral, achromatic, or gray colors, ranging from black at lightness 0 or value 0, the bottom, to white at lightness 1 or value 1, the top.
In an attempt to create an intuitive color mixing model, computer graphics pioneers at PARC and NYIT developed the HSV model in the mid-1970s, formally described by Alvy Ray Smith in the August 1978 issue of Computer Graphics. In the same issue, Joblove and Greenberg described the HSL model--whose dimensions they labeled hue, relative chroma, and intensity--and compared it to HSV.
These models were useful not only because they were more intuitive than raw RGB values, but also because the conversions to and from RGB were extremely fast to compute: they could run in real time on the hardware of the 1970s. Consequently, these models and similar ones have become ubiquitous throughout image editing and graphics software since then.
Munsell color system
Another influential older cylindrical color model is the early-20th-century Munsell color model. Albert Munsell began with a spherical model in his 1905 book A Color Notation, but he wished to separate color-making attributes into separate dimensions, which he called hue, value, and chroma. After taking careful measurements of human perception, he realized that no symmetrical shape would fit. So he reorganized his system into a blob.
Munsell's system became extremely popular, the de facto reference for American color standards--used not only for specifying the color of paints and crayons, but also electrical wire, beer, and soil color--because it was organized based on human perception measurements. It specified colors via an easily learned and systematic triple of numbers. The color chips sold in the Munsell Book of Color covered a wide gamut and remained stable over time (rather than fading). The trouble with the Munsell system for computer graphics applications is that its colors are not specified via any set of simple equations, but only via its measurements: effectively a lookup table.
Natural Color System
The Swedish Natural Color System (NCS), widely used in Europe, takes a similar approach to the Ostwald bi-cone. Because it attempts to fit color into a familiar shape based on "phenomenological" instead of photometric or psychological characteristics, it suffers from some of the same drawbacks as HSL and HSV: in particular, its lightness dimension differs from perceived lightness, because it forces colorful yellow, red, green, and blue into a plane.
Related pages
Color theory
Color wheel
Color vision
Notes
References
Bibliography
This book doesn't discuss HSL or HSV specifically, but is one of the most readable and precise resources about current color science.
Joblove and Greenberg's paper was the first describing the HSL model, which it compares to HSV.
This book only briefly mentions HSL and HSV, but is a good description of color systems through history.
This paper explains how both HSL and HSV, as well as similar models, can be seen as specific variants of a general "GLHS" model.
. Especially the sections about "Modern Color Models" and "Modern Color Theory". MacEvoy's extensive site about color science and paint mixing is one of the best resources on the web. On this page, he explains the color-making attributes, and the general goals and history of color order systems--including HSL and HSV--and their practical use for painters.
This is the original paper describing the "hexcone" model, HSV. Smith was a researcher at NYIT's Computer Graphics Lab. He describes HSV's use in an early digital painting program.
Other websites
Illustrations and summaries of RGB, CMYK, LAB, HSV, HSL, and NCS
A color conversion applet
HSV Colors by Hector Zenil, The Wolfram Demonstrations Project.
HSV to RGB by CodeBeautify.