Chameleons (family Chamaeleonidae) are a distinctive and highly specialized clade of lizards with approximately 160 species, and many have the ability to change colors.
Chameleons are distinguished by their zygodactylous feet (having the toes of each foot arranged in pairs, with two toes in front and three behind; the hind feet reverses the arrangement); their very long, highly modified, rapidly extrudable tongues that is 1.5 to 2 times the length of their body; their swaying gait; and crests or horns on their distinctively shaped heads. Most species, the larger ones in particular, also have a prehensile tail.
They are found in warm habitats that range from rain forest to desert conditions, occurring in Africa, Madagascar (59 species are found here and nowhere else), southern Europe, and across southern Asia as far as Sri Lanka. They also have been introduced to Hawaii, California, and Florida, and often are kept as household pets.
Chameleons vary greatly in size and body structure, with maximum total lengths varying from 15 mm (0.59 in) in male Brookesia micra (one of the world’s smallest reptiles) to 68.5 cm (27.0 in) in the male Furcifer oustaleti.
Many species are sexually distinguishable, and males are typically much more ornamented than the females. Different species are able to vary their coloration and pattern through combinations of pink, blue, red, orange, green, black, brown, light blue, yellow, turquoise, and purple. It takes about 20 seconds for them to change their colours.
Color change in chameleons has functions in social signaling and in reactions to temperature and other conditions, as well as in camouflage. Color change signals a chameleon’s physiological condition and intentions to other chameleons. They tend to show darker colors when angered, or attempting to scare or intimidate others, while males show lighter, multicolored patterns when courting females.
The desert-dwelling Namaqua chameleon also uses color change as an aid to thermoregulation, becoming black in the cooler morning to absorb heat more efficiently, then a lighter grey color to reflect light during the heat of the day. It may show both colors at the same time, neatly separated left from right by the spine.
Sources: Twisted Sifter, Wikipedia.