My friend once had a betta who was completely gray. He was perfectly healthy, lived in a nice clean tank, and blew bubblenests, but he just wouldn't gain any more color. As Learn To Fly said, that could just be his true color.
It's more likely the betta in the story was chilled or something, lethargic and gray. The light probably heated up the tank again, helping him get his color back. Like the others said, the only "changes" a betta's color goes through, really, is if they color up after adjusting to their new home, or if they're marbles.
When bettas become very stressed they'll lose a lot of their coloring. I bought one from Walmart that was a really dull gray color and not even an hour after putting him in a tank he turned black and red. When bettas are in an ideal environment (2Gs or more and temp that's 76-80*F) than they shouldn't lose any color. If your fish has the marble gene, they'll change colors.