Why special themes exist
What color blindness actually is
Color vision deficiency (CVD) is not grayscale vision. Almost no one with “color blindness” sees in black and white. What actually happens: the eye has three types of cone cell, each tuned to a band of the spectrum (long = red, medium = green, short = blue). In CVD, one cone type is either missing entirely or its sensitivity is shifted toward a neighbor. The result is not “no color” — it is that certain pairs of hues that look obviously different to most people collapse into the same perceived color for a CVD viewer.
Which pairs collapse depends on which cone is affected:
| Type | Mechanism | What collapses | Prevalence |
|---|---|---|---|
| Deuteranomaly / deuteranopia | M (“green”) cone shifted / absent | red ↔ green, brown ↔ olive | most common; part of ~8% of men |
| Protanomaly / protanopia | L (“red”) cone shifted / absent; red appears darker (dark red ≈ black) | red ↔ green, and red looks dim | ~1–2% of men |
| Tritanopia | S (“blue”) cone absent | blue ↔ green, yellow ↔ pink, purple ↔ red | ~0.001–0.01%, both sexes equally |
| Monochromacy | no working color vision at all | everything | very rare |
Red–green CVD is carried on the X chromosome, which is why it concentrates in men: roughly 1 in 12 men and 1 in 200 women (Colour Blind Awareness UK; NEI type taxonomy). Scale that to real audiences (per Okabe & Ito): a 250-person audience contains at least 10 CVD viewers; a product with 10,000 users reaches about 400; three random white male reviewers carry a 22% chance that at least one is colorblind.
Why ordinary themes fail
Most color themes pick colors for how they look to full color vision. Red means error, green means success, a diff’s additions are green and deletions red. For a deuteranope those two colors sit on the collapsed axis and become nearly the same muddy tone — so the one signal the UI most needs to communicate is the one that disappears. This is not a contrast problem (the colors can be equally bright and still indistinguishable), so “high contrast” themes do not fix it.
Why the standard doesn’t fix it either
WCAG does not require CVD-safe palettes. SC 1.4.1 (Use of Color) only bans color as the sole information carrier, and a ≥3:1 luminance difference counts as a non-color distinction — a page can pass AA while pairing red with green everywhere (Understanding SC 1.4.1). The themes below exist to fill exactly that gap.