It's a tough question to answer. Because we never really get to see it in any kind of daylight. I would think that it's probably black, to go along with Luke's outfit in general. It may just be very sandy and dusty from his journey to Jabba's palace, and that might account for the discoloration. Perhaps a picture of it will surface in the Hyperspace section soon? Lucas is opening up a big photo archive for everyone to check out.