It might be the way the scales catch the light, but also fish change color depending on mood and the light around them and all kinds of things...you might want to observe the fish more closely to see what the consistency of appearance is.
Well as long as he is not losing color and it is not coming back, it is probably not going to be a bad thing. When they pale out and stay that way it generally means they are sick, but he seems to be a bit of a chameleon and changes color to suit his environment or mood or something. I know that there are times when mine seem to have more color than others but not to that degree. It is not a cause for concern as far as I know and when you have fed him color enhancing foods (if you decide to) for a period of time I am supposing that his color will stabilize and he will decide what color he is going to stay and be there all the time. He just may be not well nourished and pales out overnight and needs to be fed during the day to bring his system back up to normal. Doubtful that he could be that far gone but just keep on with the good treatment and he will settle in.
Sounds like he is loving you. Have you given him a name? We do like to refer to our bettas by name here in the community and welcome them formally. They are so "human" that it just seems right. Welcome to you both.
Oh, I hadn't heard of them yet. Do you have any articles or references you could let us share. I would love to see any information you might be willing to share. I am always on the prowl for new betta info for my files. ;D