Color blindness

Inability to distinguish differences between some colors that healthy people can.

Usually of genetic nature, but might also occur because of eye or nerves damage or

due to use of some chemical substances.

There are many types of partial color-blindness, the most frequent being red-green. Other forms of color blindness are much much rarer, like blue-yellow, and the rarest being complete color blindess, where one can see only shades of grey.

Genetic color blindness affects men much more often than women.

Exact numbers vary in various populations. (Amongst Americans, approximately 10% of males suffer from some for of color perception deficiency. some more comparision values here)