well i'm not sure as to why such a thing would happen, but keep an eye on this fish and ask someone at your LFS. It's probably nothing you should worry about, but if it starts acting differently then maybe it might have some kind of disease.
I don't have any experience with cories, but I do know that some strains of Gourami get much darker when they are ready to breed. Coincidence? Dunno, just a thought.
hmm..i'm beginning to doubt that it's NOT a breeding behavior. the dark lil' cory is acting all weird. sorta just floats there. Doesn't really move at all. i can actually stick my hand in there and move him. doh. that sucks.
**UPDATE**
unfortunately i came home today. found him in the corner sideways. barely alive. so i put him in a lil' tub of tankwater, and into the freezer he went. so he could leave peacefully. never flush a semi-dead fish down the toilet. ahem.