Absolute colour blindness - seeing no colour at all, so the world appears like a black and white photgraph, is very rare and easily spotted. In these cases, the person who is colour blind may go through thier entire life not realizing it. As I said, it is not always well understood by the medical community. I personally went through a lot of physicians before I got some hard answers. The gene that causes colour blindness is hereditary through the female.
Daughters carry the gene, sons suffer from it.
The light receptors on the back of your eye consist of rods and cones. Cones are the receptors for bright light and colour. Rods are the receptors for geryscale and low level light. Cones are centred in the middle of your eye, directly behind your eye lens, while rods tend more aorund the outside. If you use averted vision during visual observing, you are actually trying to use your rods over your cones.