The colours the eye sees ...

... are made up of red, green and blue light.

These colours are picked up by 'cones' in the back of the eye.

Red light stimulates the red cones ...

... green light stimulates green cones ...

... and blue light stimulates blue cones.

These colour signals are sent to the brain to be interpreted.

If there is a deficiency in certain colour cones in the eye ...

... that colour will not be registered.

This can be detected by the use of special colour charts.