One of the best T-bones I've ever had had actually begun to turn green! :o I trimmed all the "rotten" parts off and voila! Best Steak Ever! Beef (except for hamburger) really does need some age, and any aging up to actual spoiling will enhance it. If your steak looked good the day before, and the fridge was cold, of course, it's the marinade that's discolored it.
If we're not supposed to eat animals, then how come they're made out of meat?