The vast majority of people with colour blindness can see many colours, but have problems distinguishing particular colours. Colour blindness is a most commonly a genetically inheritied condition affecting the colour sensing cells in the retina of the eye. Around 8 percent of males and 0.5 percent of females are born colour blind. In 99 percent of cases, red and green are the colours that are difficult to distinguish.

This does not generally cause any issues with the clarity of a person’s vision but may have vocational implications as some jobs (eg commercial pilots) require normal colour vision.