YELLOW! Yellow adds horsepower!
Given a choice I've bought yellow or respun the prize wheel to get yellow if I could. Second choice blue or red. I have a bright yellow TT, Altezza, Evo, and Miata LS. Unfortunately, many of the GT prize cars come in green, and green cars are bad luck in racing!
I think all the grey/black/silver cars look nice in real life, but race cars are supposed to be bright. I also like a bright color because they look better in the replays when you can see more details like grille openings, door handles, etc. because of the increased contrast. I don't really like dark wheels for that same reason.