'Colour blindness' or colour deficiency affects over 3 million people in the UK in some form or another. Approximately 10% of males are colour deficient but only 0.6-0.8% of females are colour deficient. It is caused by a deficiency of certain chemicals in the retina of the eye.

Defective colour vision can range from near-normal ability to distinguish colours, where typically the chemical for seeing red or green is slightly altered, to a high degree of confusion, where the chemical balance is considerably altered.

In the most severe cases, deeper colours as well as pale colours will be confused, particularly if the lighting is poor.