We learn at school that there are three primary colours, and the rest of the rainbow can be made by mixing these. Red light plus green light gives yellow, blue plus green gives cyan and so on. These people are called tetrachromats, whose unusual combination of genes may give them an extra dimension of colour.
Colour vision is provided by cone cells in the retina.
The way we perceive colour depends on the combination of the responses of the three different types, giving us three dimensions of colour. Colour television uses three colour guns, and colour printers need three inks to cater for the full range of our trichromatic colour perception. The genes for green and red pigment are both located on the X-chromosome. It sometimes happens that instead of having one of each, the X-chromosome has a green only, or two different greens. The red is lacking and the result is colour-deficiency.