Light entering the eye consists of many different wavelengths. The light is absorbed by tiny cells at the back of the eye called cones.
In the normal eye there are three different types of cones. Some cones are best at capturing long wavelength (red) light. Others catch medium wavelength (green) light, while other cones respond best to short wavelength (blue) light. The signals from these cones are sent to the brain where they are perceived as colour.
Some people are born with one or more cone types absent or different. This can lead to difficulties differentiating between certain combinations of colours. These colour vision deficiencies can be detected and graded using various colour vision tests.