Whatever colour they 'feel' like being at the time!
That's not as facetious as it sounds. It's a common myth that chameleons change colour to match their surrounds. Emotion, health, and temperature are the triggers for colour change. For instance,they'll become dark and flatten out their bodies to absorb heat; if sick they haven't the energy to change and may become quite pale; when feeling threatened, or wanting to mate, they can put on an amazing show of colours.