Shine a flashlight on him and see if you see any gold or rust colored "dust" on him. If he does, then he has Velvet, which would account for the color change. Velvet is caused by a parasite that lives in the water. If a fish's immune system is compromised, it can attach itself to the body. Several things will compromise the immune system, including water that is too cold. If he has Velvet, you'll need to get a heater pronto and medication. Then you will need to make sure he's kept in the dark, and a minimum of 76 degrees while you do the treatment.
It is very possible that he was sick before you got him, and is now just starting to show signs. The worse it gets, the worse he will begin to look. His fins will begin to clamp and he will become lethargic and refuse to eat. Generally Velvet starts by the gills and spreads to the rest of the body. My girl had it and she was supposed to be a purple/red color, but had completely turned a dull rust color, she was completely coated in it and it took about 2 weeks of treatment and dark, warm water to cure her.