It means a lower than normal ability to distinguish colours and shades. It is not common
It is usually hereditary but injury and conditions caused by side effects of certain medicines can also impair the function of the colour-sensitive cone cells in the retina, which send the brain the correct colour signal.
The inability to distinguish between red and green is by far the most common and people are generally unaware of their colour deficiency condition.
It is vital that every child should be tested for colour deficiency by age of five as much time may be lost investigating learning problems which could result from colour deficiency.
Colour deficiency may preclude certain career choices e.g. pilot, electrician
Colour deficiency can easily be revealed during and eye examination.