I have had two betta fish. One of them I had for two years and he never changed colors. The one I have now has never changed colors. I've had him for about 4 years and never once has he changed colors. He could have changed colors because you switched food. But betas don't usually change colors...
My betta does this too. I had him for about 4 years. I think it is normal. My betta fish will stay at the bottom of the tank and then go to the corner of the top. I think it is normal.