Colour blindness is a malfunction of the retina, which converts light energy into electrical energy that is transmitted to the brain. This conversion is accomplished by two types of photoreceptor cells in the retina these photoreceptor cells are the rods and cones. The cones are responsible for encoding colour. Each cone contains structures or usual pigments sensitive to one of the three wavelengths of light: red, green and blue. There are three types of cones - blue cones, red cones and green cones each one is more sensitive to its colour. Normal persons are able to match all colours of the spectrum by the mixture of only three fundamental colour sensitivities. Hence, the huge variety of colours we perceive stems from the cone cells response to different compositions of wavelengths of light.
Defects in colour vision occur when one of the three-cone cell colour coding structures fails to function properly. One of the visual pigments may be present and function abnormally, or it may be absent altogether. There are people with red-green colour blind and the medical term is Red area colour vision deficient. Such people do not have enough red and green cones. Sometimes, one of these may be absent too. Such persons often confuse red, green and brown colours and blue and purple.
However, this again, depends on the intensity of the colour and brightness of the light conditions at that time. For instance, a red-green colour vision deficient person can see the bright green crayon as green only if it is seen under bright sunlight but the same light green crayon looks as brown crayon under 40Watt lamp bulbs and the object is not as colourful as a bright green crayon but still it is green, it appears as red to the colour deficient person. Red-green colour vision deficiency is a hereditary genetic disorder, which affects one-in-twenty white males. This rarely affects the females and non-whites.
Total colour blindness is a defect where the person will not be able to see the colours, but all objects look black and white to him. This is a very rare case among all sexes and races. People with blue deficiencies are very rare. Colour blind people will have difficulty in identifying the proper colours, sometimes even on gadgets, to find the red button for stop red -signal and green-signal of traffic lights on some electrical gadgets to switch off after charging is complete (because green cannot be seen).
Colour blindness does have some advantages. Colour-blind people look for outlines and so they do not get easily confused by camouflage. That is why, colour blind people were used in World War-II spy planes to spot camouflaged German camps. Normally, the night vision of colour-blind people tends to be much better than average. Colour-blind people cannot discharge duties, which need proper colour identification like police, fire brigade, medical profession, electronic component identification, etc. Defective colour vision may be acquired by a sort of eye disorder to, but the vast majority of colour-blind cases are hereditary i.e. present at birth. Colour blindness is rooted in the chromosomal differences between males and females. Females may be carriers of colour blindness but males are more commonly affected. About 8 percent of males and 0.5 percent of females are colour deficient.