Can you post a pic? I was worried about mine a while back. Their plastrons were getting darker around the edges. It was to the point where I couldn't really tell if it was pink or a funny orange colour. Now both my res have orange plastrons. Vet says it's from beta kerotine in their diet.
What are you feeding? How old is he? Is it just a slight change in color or does it look like blood under the shell?
Here's what I found. Difficult to tell because it's a snapping turtle but there is some info there.
http://www.austinsturtlepage.com/Care/medinjuries.htm