Color blindness, also called color vision deficiency, is a reduced or altered ability to distinguish certain colors. It affects roughly 1 in 12 males and 1 in 200 females, so a color choice that seems obvious to one viewer may be ambiguous to many others. Inclusive visual design treats this as a normal audience need, not a rare edge case. section 4.1.4 section 4.4
Color perception depends on three cone-cell types that respond most strongly to short, medium, and long wavelengths—often called blue, green, and red. With all three working, vision is trichromatic. A nonfunctioning cone type produces dichromacy; two or more nonfunctioning types produce monochromacy. Complete absence of functioning cones is achromatopsia. section 4.1.4
Common color vision deficiencies change which distinctions are dependable:
- Protanopia is red-blind dichromacy, relying on blue and green light; deuteranopia is green-blind dichromacy, relying on blue and red light. section 4.1.4
- Tritanopia is blue-blind dichromacy, relying on red and green light. section 4.1.4
- Protanomaly and deuteranomaly involve shifted or weaker red or green cone sensitivity; tritanomaly makes blue–green and yellow–red distinctions difficult. section 4.1.4
- Deuteranomaly, where green sensitivity shifts toward red, is the most common type discussed here. section 4.1.4
Why traffic-light colors can fail
Red, yellow, and green often signal bad, caution, and good, but red and green can be difficult to separate when red or green cone function is affected. For a diverging heatmap, orange, gray, and blue make the categories easier to distinguish under deuteranomaly and deuteranopia simulations. If traffic-light colors are required, separate red and green spatially, use clearly different lightness levels, and avoid many shades of each color. section 4.4.1
Do not use hue as the only carrier of meaning. For continuous scales, vary lightness as well as saturation so values remain distinguishable in grayscale—“get it right in black and white.” A two-hue gradient can retain visible color at both ends while lightness still communicates lower versus higher values. Check contrast against the background, especially for low values that may fade away. section 4.3.1
Palette generators can provide a starting point, including accessibility-oriented options, but generated colors still need inspection. In categorical palettes, verify that neighboring colors differ enough for viewers with color vision deficiencies; for example, orange and red may be too similar for some red deficiencies. Adjust saturation or lightness when a color draws unintended attention. section 4.3.3
Figure 4.33 The top row shows a classic traffic light palette applied to a heatmap and what it looks like to someone with deuteranomaly (center) and deuteranopia (right). The bottom row shows a more colorblind-friendly palette of orange, gray, and blue instead of red, yellow, and green, respectively, under the same simulations. Remember, deuteranomaly is the most common type of color blindness, where the peak absorption of the green cone shifts toward the red end of the spectrum making greens appear more red. Deuteranopia is when the green cone isn’t functional at all.
Compare a red–yellow–green heatmap with orange–gray–blue under green-cone-deficiency simulations to see why hue alone is a fragile signal. section 4.4.1
