I'm thinking a burgundy or deep red would go very well with your grey interior. There was a gorgeous Charger at MATS a couple of years ago that was that color combo, including the vinyl top and rear stripe and it was very striking. I'll see if I can find the pic and post it.
(12/10) Found the pics from the 2009 MATS. I think this is a beautiful color combo...and not one you see everyday
