Colour production in nature may be chemical or physical. Chemical colours are produced when certain of the component rays of the beam of white light are absorbed while passing through a medium allowing others to be reflected to the eye.
Physical colours are produced by refraction of a beam of light into its constituents rays when it falls on a prismatic surface.
(a) Chemical Colours
Pigments occur in all the parts of an animal on the surface as well as in the deeper parts. These pigments are melanin. When the pigment is external it gives colour to the organism.
The colour is of real value in the struggle for existence. Pigment and the cell, which possess it, are differentiated into plain pigment that is responsible for the unchangeable hue to the animal, and chromatophores or changeable pigment spots.
Chromatophores are the cells which have radiating fibres lying in a Plane parallel to the surface of skin. During relaxation, a mass of pigment lies deep and thus a small visible area is present.
When the fibres contract, the pigments spread over a great part of the surface. And this is manifest to the eye. A good example of this phenomenon is Chameleon.
Colour changes are also produced by chromatophores influenced by the eyes and skin through the pituitary gland; for example in African Chameleon or American lizard, known as Anolis.
(b) Physical Colours
Production of physical colours is dependent on the surface structure; light falling upon which is reflected by finely incised parallel lines, often running in more than one direction and thus undergoing dispersion into its component rays. The examples are found in beetle’s wings, scales of butterfly and feathers of humming birds’ throats.
In Morpho, the tropical butterfly, the colour changes from blue to the greenish hue, though the actual colour is brown. The colour of the neck of some pigeons changes from brilliant metallic red to lustrous green.