The pathological variations in colour in the tissues come under a considerable number of different categories.

The endeavour has been frequently made to refer all pigments found in the body to the blood-pigment, but this relation has been definitely disproved for some (as in the case of pigments introduced from without), and rendered doubtful in others. The normal pigments in the body, those of the blood-corpuscles, of the skin, eyeball, etc., arise in cells and are formed by the cells elaborating them from the nutrient material afforded them. It is an assumption to suppose that the pigment of the skin is derived from the blood-pigment and not elaborated by the cells.