if you have a dark gray color carpet then u need to have a sofa in beige color and if the carpet color is light gray then u can have a black sofa as well as well beige as well. Both works
A dark blue or black one might look nice... or perhaps a red or maroon tone. You might also want to think about beige or taupe, which goes with everything.
It sounds like the room is fairly neutral now, so almost anything should go just fine. Avoid most browns, as they typically crash with gray. I am a big fan of reds with gray, but it all comes down to what you like and how strong you want the colors.