He looks to be a solid colored betta in your first pic and marbles generally start out white and then slowly develop color over a few months-at least from what I've seen. So the reddening in color could be due to some ammonia burns. Keep his water clean and warmed with a heater and it should get better. c:
Though I'm no expert, so posting in the diseases and emergencies subforum might be a good idea. If you do end up posting in there be sure to fill this (http://www.bettafish.com/showthread.php?t=49233
) out when you make a new thread.