Probably a shed. Most likely a shed. But - the other thing it could be (skin color changes are usually a shed, a stain or a burn) is a burn. What kind of sun lamp and how close is it?
In the winter, when the temps are cooler, chameleons tend to bask closer to their bulbs. All my chameleon burns (if not all, almost all of them)took place in the winter.
Usually, you'll see very pale, "glowing" white skin, then it'll turn black sometimes. Bad burns are sometimes black outright.
If the skin is loose, and translucent (especially when soaked with a sprayer), it's a shed. If it looks normal except for the color (and seems to glow at night - when he's sleeping and you put a light on him, of course) it may be a burn. Take a pic (in good focus, blurs will make it useless) as close as you can get, and post it.
Another thing to do is take the temp where he basks. See how hot it is. A quick test to to put your hand near the light (as close as the chameleon can get in his cage - even if he's got to climb upside down), and see if YOU get burned. If you can't take the heat, it's a potential burn hazard.