Some colouration is definitely due to the chemical material of the organism, while the hue is completely due to the absorptive power of that chemical material.
For example, iron oxide is red, therefore, blood of the vertebrates is red and since copper oxide is blue, therefore, the blood of Octopus is blue. In several cases, colour of internal organs is due to the nature of the food taken. Bones of garpike are green due to the disposition of green vivianite under some physiological conditions.
Thompson regarded that pigment substances are mainly waste products; reserve products or by-products of the animal’s metabolism and in many cases, the colours have no more significance.
Pigments, besides protective or warning significance, may have other importance also. For example, definitely localised spots of colouring matter are, sometimes, associated with end organs of the nervous system. This pigment absorbs light, indirectly stimulating the nerves and thus serves as a light recipient organ. All eyes works on their basic principles but in some cases, accessory organs have a light perceptive value.
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Other pigments found in feathers or fur may absorb or reflect heat. Thus these pigments are valuable to the owner in addition to the function of sympathetic colouration. Dark pigment such as present in Nigro and other dark human races serves to cut off the ultraviolet rays of light. Hence a coloured man under a tropic sky is less liable to sunstroke than a fair one.