Medical Term for Colour Blindness: Red-Green Colour Vision Deficiency
Red-green colour blindness is more correctly referred to as "colour vision deficiency" since the sufferer is not entirely blind to colour.

The visible spectrum:

In red-green colour vision deficiency, the visible spectrum is divided into two parts; a red segment and a blue segment, seperated by grey or indistinct areas. The amount of grey or indistinct areas varies according to the severity of the deficiency. There are four types of red-green colour vision deficiency:

Protanopia: Blue-green appears grey, red-purple appears grey.

Protanomalia: Blue-green appears indistinct greyish, red-purple appears indistinct greyish.

Deuteranopia: Green appears grey, purple-red appears grey.

Deuteranomalia: Green appears indistinct greyish, purple-red appears indistinct greyish.

Approximately one in twenty European Caucasian males have a measurable red-green colour vision deficiency. It is exceptionally rare in females or races other than European Caucasians.
 

Total Colour Blindness
Total colour blindness is rare, but can be found in both sexes and all races. It mainfests itself in two types:

Typical: Complete failure to discriminate between any colour variations, usually associated with other severe vision impairments.

Atypical: Colour sensitivity is so low that only very clear colours are perceived, but there is little other visual impairment.