from what i understand, color changes can occur from stress (they would be more pale), temperature changes (more vibrant when introduced to higher temp water), and exhaustion.
I have noticed all but the third in my own betta who paled after being introduced to a new tank, fearing the new environment. He also gained some blue tints in his tail and fins after having a heater put in raising the temp from 74 to 79.
Don't listen to all the people in the pet store world telling you all the ways a betta "can" survive. A human being could survive in a 1 room shack, with no central air and a cup to pee in, but that doesn't mean it would be a pleasant existence. I still argue every day with my girlfriend about what NEEDS to be done to ensure the health of our betta. She calls me obsessed and over worried when I say "we need to do a 30% water chance tomorrow". Heed all the advice you get here from senior members.