Chameleons are probably best known for their ability to change the colour of their skin, though this ability is not so much related to the colour of the surface on which they are standing, for camouflage, which has widely been assumed, but more on their mood, health and temperature.

Colour changes happen when breeding, excited, under different lighting conditions, temperature or in the presence of another chameleon.

This male Jacksons chameleon was hanging upside-down from the shade netting

From about the age of 5 - 6 months chameleons begin to develop the ability to change colour from the more subtle colours of dull grays to those that are brighter - green, turquoise and blue-green. 

The colour of their skin changes by the movement of pigments (or colours) in the skin, and the pigment most responsible for the changes in colour is melanin. 

Melanin is a dark pigment which is stored in melanophores, and when the melanin are concentrated in one area inside the melanophores, the bright yellow and blue pigments can be seen in the other skin cells (which mixed together make the colour green), and therefore the chameleon usually looks green.  When the dark melanin spreads out among all the cells they darken and the chameleon appears to be brown or black, and even sometimes stripped or spotted!

Chameleons can not decided for themselves to change colours, this is controlled automatically by their hormones and nervous system.

Photos  courtesy of 'Chameleons' by Claudia Schnieper and Max Meier

These are some of the colours into which just one species of chameleon can change. 

Photo  courtesy of 'Chameleons' by Claudia Schnieper and Max Meier