Something in maroon. I'm quite into the Peugeot Diablo red atm (for obvious reasons!) contrasting with the dark grey carpet, black leather/grey alcantara seats contrast well with the maroony red trimmed GTi carpets on the floor. The wing mirrors look cool black/diablo red too.
Maybe you can use that shade of red somewhere in contrast to the dark grey of the car, and black of the interior? The wood trim (that's painted white in the pics above) would look a bit bleurgh wholly in red, maybe black, with a discreet red edging, perhaps a horizontal pinstripe or two?