I think if anyone knows about custom teams, it would have to be the tuner car crowd. For example: Autobacs has that reddish-orange color on their cars, Signal Auto has that nice orange color kind of perfect for Fall/Autumn colors, and who can ever forget the hot pink Yashio Factory machines? In terms of real race cars (not bashing tuners), you have the lovely silver colors for Mercedes-Benz, the white and aquamarine colors of Konrad Motorsport (or is it "Konrod?"), the former sky blue colors of The Racer's Group Porsches (you can paint up Ruf RGTs and make your own racer), and even the red and white of Holden Racing Team (HRT). The game "Ridge Racer V" has a special paint scheme featuring three different colors and an fourth alternative color. So you basically have your race team with the six Standard and six Extra cars.
Personally, I'd love to make my own team colors. I won't talk about my "team colors," so I'll leave it at that.