Bloodworms may work but my betta's wouldn't touch them and it made my water gross so be careful with those! I ended up throwing a whole package out and it was a waste of money.
I think that betta bites are pretty cheap and don't have any color enhancing stuff. But I am not sure if there really is any color enhancing things in the food besides them being good for the fish which makes them healthy and then they show off their colors. So your fish might have be undernourished if you want it to stay white!

NOt sure though. GOod luck. Also I know that natural light helps their colors so if you keep them out of the sunlight maybe the colors will go away?
Hope this helps.