With forest green carpets, I'd do a white or taupe on your walls. You don't want to choose a shade that is too dark, as the carpet is already dark enough.

Another alternative is to lighten up the rug area by adding a large area rug. If you buy one large enough you could cover a big enough area that most of the green is covered. A contemporary area rug like this Anji Mountain Bamboo rug would jazz up your room and detract from the forest green rug.