What causes a person to see only one color?

People do not usually only see one colour. People can experience red-green colourblindness, which makes it difficult to distinguish between red and green. This is caused by a mutation in the genes that code for the proteins that detect red and green light in the retina. In rare cases, people may have complete colourblindness, also called monochromacy, in which they can only see shades of one colour. Monochromacy is caused by an absence or mutation of the light-sensing cone cells in the retina, which are responsible for colour perception.