The problem of colour-blindness

Colour-blindness manifests itself as an inability to distinguish reds and greens. It results from problems with cells on the retina that are receptive to red or green light.

Full colour vision relies on the smooth operation of 3 types of cell that are sensitive to colour light: blue, red and green cone receptors. People who are colour-blind generally have a problem with the red or green receptors. They are either missing, or not tuned in to the right colours 1. The result of this is that colour-blind people have trouble distinguishing between reds and greens.

Colour-blindness can be divided into various types. Some colour-blind people have problems with their red, or 'rho', receptors and are known as protan - they can either be red-blind (protanope) because they do not have red receptors, or red-insensitive (protanomolous) because their red receptors are not properly tuned-in to red light. Others have problems with their green, or 'gamma' receptors and are known as (deutan) – they can either be green-blind (deuteranope) or green insensitive (deuteranomolous). For both protans and deutans the severity of colour blindness can vary from total red or green blindness, to mild insensitivity.

Green and red blindness compared types of colour blindness 1

Colour blindness type

receptor name


wavelength of peak receptivity


proportion of colour blind population

Red blind (protan)






Green blind (deutan)




yellowish green


One very rare form of colour blindness is achromatopsia, in which none of the colour receptors are functioning. This only effects 0.003% of the population, however the simulation of this condition that EveryEye provides is usefull in checking for visibility across all types of colourblindess.

Why colour perception is important in design

Colour has many uses in online design. It is used to provide emphasis, indicate functionality, characterise content, produce visual hierarchies of information and create visually pleasing designs. If the colours are not carefully chosen they will not achieve their intended purpose when viewed by colour-blind users making your site harder and less pleasant to use.


1. Rigden,C. 'The eye of the beholder' - designing for colour-blind users. British telecommunications engineering 17, 2-6 (1999).