You know, that sort of thing seems to be sort of common with scaled animals. I don't know how different it is for fish, but in reptiles there is such a thing as 'firing up', where at certain times of the day the colors will be brighter or even completely different!
For instance, this is the same snake: one picture taken at night, the other was the next morning
No she wasn't in shed, in fact she had shed right before the 'dirty' looking picture
Or, this gecko will change drastically when fired up:
Also, hormones will change the colors of a snake. A green tree python that is pregnant will turn blue, while a pregnant ball python will "glow" about 10 shades brighter than her usual colors. I wonder if daily activity or hormones relating to seeing males or other bettas could affect their color slightly or the way it reflects in the light.
This is all me guessing around here. I'm sure it's something very different, but it just reminded me of these examples. :)