The colour of frog skin varies from greyish brown to olive green, generally with yellowish brownish or blackish patches. The capability of change in skin colour is called, metachrosis. The ectodermal chromatophores (pigment cells) contain numerous minute granules in their cytoplasm. All colour changes are under hormonal regulation. These help the frog in avoiding detection by enemies and preys. Skin colour in vertebrates is mainly due to melanin pigment. In fishes, amphibians and reptiles granules of melanin occur in special cells termed chromatophores. In birds melanin occurs only in feathers and in scales of hind limbs. In rabbit and other mammals it occurs in certain highly branched cells called, melanocytes.