I noticed every time I put my Chameleon under the sun he gets lighter & starts to show a lot of red. I'm assuming he loves the sun compared to any light.
Of course he loves the sun, it is part of his natural habitat!
From my understanding, chams tend to turn lighter colors in hotter temperatures (from better sources of light like the sun), so I suppose that's normal. If he is constantly showing dark colors and only light colors outside, you may have a lighting/temperature issue in his enclosure as he may be staying darker colors to absorb more heat from the light available.