Colour blindness

A condition affecting the perception of colour, caused by absent or altered photoreceptors in the retina. The main types are classified by which colour receptors are affected: protanopia (absent red receptors), deuteranopia (absent green receptors), and tritanopia (absent blue receptors), with milder anomalous forms (protanomaly, deuteranomaly, tritanomaly) where receptors are present but have altered spectral sensitivity. Monochromacy (complete colour blindness) is rare. In digital accessibility, colour blindness is addressed by ensuring colour is never the sole means of conveying information (WCAG 1.4.1), maintaining sufficient contrast ratios, and using patterns, labels, or icons alongside colour coding. User capability models can represent colour blindness as percentage values of effective colour perception across frequency ranges, providing quantifiable input for adaptive interfaces rather than relying on lists of specific colours to avoid.