I think that you will have to give a bit more information. What kind of yellowing, on the skin, slim, what part of the body. When did this occur? Has anything in the tank changed? Any other symptoms? You know 5W's and ....
They are all blue to begin with. I had one that slowly turned yellow. It means its a male. He was one bad dude too. He ended up being completely bright yellow, and I will tell you his personality was anthing like his color. He ruled my 125 and would bite the fire out of me when I would stick my hand in the tank. Sweet fish.