Colour blindness, or colour vision deficiency, is the inability or decreased ability to see colour, or perceive colour differences, under normal lighting conditions. Colour blindness affects a significant percentage of the population. There is no actual blindness but there is a deficiency of colour vision. The most usual cause is a fault in the development of one or more sets of retinal cones that perceive colour in light and transmit that information to the optic nerve. This type of colour blindness is usually a sex-linked condition. The genes that produce photopigments are carried on the X chromosome; if some of these genes are missing or damaged, colour blindness will be expressed in males with a higher probability than in females because males only have one X chromosome (in females, a functional gene on only one of the two X chromosomes is sufficient to yield the needed photopigments).
Colour blindness can also be produced by physical or chemical damage to the eye, the optic nerve, or parts of the brain. For example, people with achromatopsia suffer from a completely different disorder, but are nevertheless unable to see colours.
Colour blindness is usually classified as a mild disability, however there are occasional circumstances where it can give an advantage. Some studies conclude that colour blind people are better at penetrating certain colour camouflages. Such findings may give an evolutionary reason for the high prevalence of red–green colour blindness. And there is also a study suggesting that people with some types of colour blindness can distinguish colours that people with normal colour vision are not able to distinguish.