Hi, and welcome to the community.
How long have you had your Betta?
Has his other colors intensified since you got him? It could be his pattern coming out since he is getting more comfortable. Or maybe not...
Is he eating alright, does it seem to bother his mouth opening and closing?
New to FW fish? First tank in a long time or EVER??? Need some info on keeping fish?
Start here - Aquarium Advice Article
FW Favorites right now Clown Loaches, any shrimp - SW Vase Red Mangroves