## 2 Materials and Methods
### Equipment  
The experiment was conducted in a controlled, quiet laboratory at the University of Zagreb, Faculty of Graphic Arts, with an environment carefully designed to minimize distractions, reduce background noise, and focus participants' attention by eliminating distinct light sources and positioning test materials directly in front of them. For reading on screen, the Tobii Eye Tracker X60 (Tobii Technology, Stockholm, Sweden) was used to collect data, operating at a sampling rate of 60 Hz with an accuracy of 0.5 degrees.  
Figure 1: (**a**) Sample with an achromatic (black) text on an achromatic (gray) background—AA; (**b**) sample with an achromatic (black) text on a chromatic (green) background—AC; and (**c**) sample with a chromatic (purple) text on a chromatic (green) background—CC.  
The device was used to measure the time that the participants spent reading, with the measurements specifically targeting the textual area defined as the area of interest (AOI). Three promotional texts, as well as three multiple choice questions after each text, were presented on a computer monitor (model LEN L1900 pA, Lenovo, Morrisville, NC, USA) with a screen resolution of 1280 \(\times\) 1024 pixels. The Tobii Studio 3.2.1 software was utilized for the presentation of stimuli.  
For reading on paper, the colors used in the text and background combinations (black, gray, green, and purple) were measured in accordance with the CIELAB color space. The samples were printed using a digital printing press, Minolta AccurioPrint C3070L, on Kunstdruck matte paper, 170 g/m\({}^{2}\). The printed color values for \(L^{\star}\), \(a^{\star}\), \(b^{\star}\), and \(C^{\star}\) were measured using an Ocean Optics USB 2000+ spectrometer under D50 illuminant and 2\({}^{\circ}\) standard observer conditions (values shown in Table 1) to determine the color difference that was critical for legibility.  
Reading time for the printed stimuli was measured under standard lighting conditions using the Macbeth Judge II light booth (X-Rite, Grand Rapids, MI, USA), with an X-Rite D50 light source to ensure consistent lighting for all participants. According to the ISO 3664-2009 standard [33], it typically takes a few minutes for an observer to visually adapt to a new light source in a viewing booth, ensuring accurate color perception under controlled lighting conditions. The stimuli were positioned at an angle of approximately 45\({}^{\circ}\) within the light booth.  
CIEDE2000 [34] is a modern color difference formula developed by the International Commission on Illumination (CIE) to improve accuracy in color difference assessment compared to previous formulas like CIE76 [35]. It uses parameters such as hue, chroma, and lightness to more precisely determine how different two colors are. When used to measure the color difference between text and background, CIEDE2000 provides valuable insights into how visually distinct the colors are. [PERSON] et al. [23] noted that for optimal accessibility, a noticeable color difference is recommended, as higher contrast improves text clarity, even for users with impaired vision or color perception difficulties. As the CIEDE2000 value increases, the contrast becomes more pronounced, enhancing the text's visibility.