I agree with admin. Especially if this boy was previously exposed to a lot of sun - you said he was sunburned when you got him, right? Sunburn would definitely make his skin darker, just like it does with humans. The skin in the wrinkles wasn't exposed, so it is lighter. You may find that it fades in time. But I would say this looks perfectly normal. Even my cats are a bit lighter in their wrinkles, though not this much.