I have a betta that turns a variety of colors.....he is a greenish on his body, and his color graduated to a bluish to a red on his fins. WHen he was in my 55 gal tank he was mainly blue all the time. Now he's more of his original color. He varies as to intensity of color, and what % of him is what color. Sometimes, the front of his face is white.
I have a red one, and so far I've not seen him change at all.
I would say as long as it's not fuzzy white, which would be fungus, it may just one of his color variations.
MAybe someone cal tell us how and why they change colors.