Question 1

How common is color blindness?

Accepted Answer

About 1 in 12 men and 1 in 200 women of Northern European descent have some red-green color vision deficiency. Blue-yellow deficiency (tritan) and complete color blindness (achromatopsia) are rare.

Question 2

What is the difference between protanopia and deuteranopia?

Accepted Answer

Both are red-green deficiencies. In protanopia the long-wavelength (red) cones are missing, so reds also look darker. In deuteranopia the medium-wavelength (green) cones are missing; reds keep their brightness. The milder forms, protanomaly and deuteranomaly, are far more common than the complete ones.

Question 3

How accurate is a color blindness simulator?

Accepted Answer

It is a model of an average viewer, not a picture of what any one person sees. This page uses the Machado, Oliveira and Fernandes (2009) model, which is widely used in design tools. Use it to find information carried by color alone, then fix that with labels, patterns or contrast rather than tuning colors to the simulation.

Question 4

Does passing WCAG contrast make a design safe for color blind users?

Accepted Answer

Contrast helps, because luminance differences survive every type of color blindness. But WCAG 1.4.1 also requires that color is never the only way information is shown, such as red and green status dots with no label. A simulator is the fastest way to catch those.

Type	What's affected	Roughly how common
Deuteranomaly / deuteranopia	Green cones weak or missing. Reds, greens and browns get confused.	About 6% of men (mostly the mild form)
Protanomaly / protanopia	Red cones weak or missing. Reds also look dark, so red text on black nearly disappears.	About 2% of men
Tritanomaly / tritanopia	Blue cones weak or missing. Blue and green, and yellow and pink, get confused.	Under 0.01%, equal in men and women
Achromatopsia	No working cones: only light and dark.	About 1 in 30,000

Color blindness simulator

The types, and how common they are

What to fix when something disappears