to me it looks like there isnt enough cover for your cham to hide in. if he doesnt feel safe and hidden he might get stressed out and brown like that, it might be a simple fix of getting a lot more cover in there for him. i think the cage should have some open areas but mostly cover for him to hide in. dont make it to crowded so he cant move around just enough to feel safe.
also i have once changed my uvb bulb to a 10.0 because i figured that the screen filters about half or so and then if he isnt within 6 inches or so he doesnt get 100% of the uvb rays, but when i switched my cham seemed to be browner and more stressed. sence i switched back he is back to green. its just my experience, but if you decide to get a 10.0 watch him closly.