A person with achromatopsia, or total color blindness. The congenital pathology, caused by recessive genes, is extremely rare, affecting in the normal course of events something like 1 in 30,000 individuals. Because achromatopes also have trouble with seeing fine detail, for reasons I've explained in the achromatopsia mode, they have trouble reading and may not receive an adequate education. Achromatopes tend to squint in bright light, and usually wear very dark wrap-around sunglasses except at night. Their eyes may jerk continually, and they will require magnifiers to be able to read. They do not see everything as grey, exactly, but rather a world of differing luminances. For example, they can see patterns in weavings using yarns of little chromatic contrast which would be invisible to the normally sighted.