Could you take a picture of it? It could just be his natural fin color, or if he's also breathing heavily and has other symptoms, he could possibly have a gill infection. There are multiple causes including a bacterial infection and ammonia poisoning (red/velvet gills). However, if it's a tannish color, could be nitrate poisoning. According to the petco guy, there really isn't much you can do if it's ammonia poisoning as if you use a strong ammonia eliminator it could shock the fish even further. I used Kent Betta Bowl Essential (the only conditioner/primer I saw at petco that also eliminated Ammonia). He's breathing easier now. If it's bacterial try a medicine that is a broad spectrum cure, bettatalk who used to have more specific cures unfortunately closed its doors but recommends using a broad spectrum antibiotic. If you can't find one (like me) I'm using Tetra Lifeguard which is supposed to be broad spectrum as well (although definitely not as effective as an antibiotic).
Again, if you could post a picture, that would be helpful as it's probably a bad idea to try curing him if he's not really sick. Good luck!