Marbles can be any combination of colors and they can range from being mostly one color with a couple spots of another color here or there, or they can be 2 or 3 different colors all over. They can change color as they get older.
Edited to add: Check out this thread for a couple pics of hers and how it changed.
Yeah, sometimes it's just like... they're chameleon fish. It was just my boy's body that changed color, but I could definitely see just the fins changing color, too. Don't worry. Just let your marble boy be super cool!!