My baby bettas who are probably now about teenagers
are a constant kaleidoscope of color! They change from light to dark, blues to greens to purples, sometimes I can't even decide which is which. They get horizontal stripes and vertical bars.
So here's what I know. Sometimes the color change is at night. Mine change during the pecking order establishment and the colors can change minute by minute. Lighting changes everything! Taking pictures with a flash shows colors where the eye can see none. My adult males have never changed colors. The horizontal stripes occur in the young ones and also when older ones are stressed. The vertical bars occur in females who are ready to breed. One I think is a male is showing vertical bars. I believe this is a submissive sign.
Did I cover everything appropriately? I don't think that the fin colors change as drastically, if at all, just the body colors.