Contrary to the popular misconception, chameleons do not change colour to match their surroundings. Several factors contribute to the colour change. Light, temperature and emotional state commonly bring about a chameleon's change in colour. The chameleon will most often change between green, brown and grey, which coincidently often matches the background colours of their habitat.
The chameleon has several cell layers beneath its transparent skin. These layers are the source of the chameleon's colour change. Some of the layers contain pigments, while others just reflect light to create new colours. So the colour of the chameleon in the mirror would, most likely depend on what mood it was in.
Another interesting fact about chameleons is that they can move their eyes in two different directions at the same time! Creepy, huh?
See: http://win.kidzworld.com/site/p1740.htm copy