The Temperature Around
It is a known fact that dark colours absorb light and thus heat faster whereas light colours reflect it. This is the reason why we prefer wearing lighter shades during summers and darker ones during the winter season. Surprisingly, chameleons also seem to know this and thus use their skin as a thermostat so as to maintain their body temperature. When a chameleon feels cold it changes its colour to dark green. Similarly, when it is hot outside, it changes its colour to mint green.
One study shows that chameleons change their colour as per their mood. This is to say that the colour they don is a means of self expression. When scared or angry, these creatures darken their colour and when excited they change their colour to a brighter shade.
Chameleons are also said to change colour for the purpose of matting. They don lighter shades to attract mates. It has been observed that male chameleons change their colour more often as compared to the females who give other subtle hints for this purpose.
Certain studies also state that chameleons change their colour based on the intensity of light. A chameleon may change its colour several times a day based on the above stated factors.