Hmm, only things I can think of are these...
1. PH, or hardness of water - including metals, can cause this discoloration - although not extremely dangerous.
2. Food and water: this affects how a betta looks. A good diet, and clean water will have colors show up they never had! Same with bad diet, and dirty water.
3. Ammonia burn. Check inside his gills when he flares. Pink? good then scratch ammonia burn... black or inflamed red? ammonia burn is a possibility. To avoid this, let the tank cycle... do a smaller water change (30% per week).
4. bettas change colors. Like I said, one of mine has baby blue odd discoloration on his cheeks - but that's just him.