My best answer matthew is natural sunlight especially how sunny it is now. Ive been getting mine out as much as posible, and if he wont let you take him out, take out his cage.
Mine tend to do darker browns and beiges but as soon as I take them outside they brighten right up! One of them does a beautiful green with diamonds on his back and the other goes red/orange. They're only ten weeks old so I can't wait till they fully develop their colors
All my chams look pretty bland inside their cages.
They dont color up until I let them out, or feed them and they are hunting down their food.
My panther always looks dark grey in his cage, but when he is crusing the free range, he lights up like a christmas tree