Discoloration is gunna happen to all pipes to varing degrees. Should really show about half way up head pipe. Will taper off after that. However, i wouldn't worry about the discoloration to much. Its gunna happen. As for the back fire, this is a fuel/air issue. As stated above try adding lid back on 1st. Do is back fire when you give or release gas? Any smoke...color? 1 pop or many in succession? What EFI you running?