Yeah, bettas have charachter I love 'em
They can change their appearance a bit - become duller/paler when frightened or trying to hide etc, brighter when showing off, but i don't think they would actually change colour? I think it's more likely that it's the amount of natural light changing that makes different colours show more.
I have a male that has dark blue, bright blue, purple, greenish and aqua, and i often can't see some of the colours, but when i get him under a decent display light i can see all of them - just depended on the time of day/amount of light/direction of light.
Hope that helps