Chameleons will oftentimes display more brownish tones than you'd expect from looking at characteristic pictures of the different locales. When they aren't in a state of emotional perturbation, they tend to look more subdued like you see here. And, like everyone else said, they turn darker to bask more efficiently.
When's the last time he shed? It looks to me like that dulling of colors could be characteristic of a shed about to come on