Yes, it's ok that he's changing color. Many fish do that when coming home from a pet store and adjusting to their new healthy surroundings. Often, pet store bettas are very pale or a completely different color than what they are supposed to be due to stress, cramped homes, and usually unclean and cold water.
He probably is seeing a little of his reflection in the corners, but not enough to flare all the time (which is good). My female will give me a small flare when see sees me, almost like a greeting. She is very aggressive though, so when she sees a reflection, she will give a full out flare and try to attack. My male has yet to flare at anything, but he's still young.