Textbook of Usability

Also known as: colour blindness, CVD

Colour vision deficiency (commonly called colour blindness) refers to conditions that impair the ability to distinguish certain colours. The most common form, red-green deficiency, affects approximately 8% of males and 0.5% of females of Northern European descent — roughly 1 in 12 men.

Human colour vision is normally trichromatic, based on three types of retinal cones sensitive to short (blue), medium (green), and long (red) wavelengths. Deficiencies arise when one cone type is absent (dichromacy) or has an abnormal response curve (anomalous trichromacy):

• Protanopia/protanomaly: red cones absent or altered
• Deuteranopia/deuteranomaly: green cones absent or altered (most common)
• Tritanopia: blue cones absent (rare)

The design rule is simple and absolute: never use colour as the sole channel for conveying information. Always provide a redundant cue — shape, pattern, position, or text label. A traffic-light dashboard becomes inaccessible to red-green colour-blind users unless the indicators also differ in shape or include text.

This principle is codified in WCAG 2.1 Success Criterion 1.4.1 ("Use of Color") and is a core tenet of accessible design. Designing for colour-blind users is not a niche accommodation — at 8% prevalence in men, it is mainstream.