I would use a neutral color for sure. Those are colors that seem to go with anything, and include whites, grays, silvers, and blacks. Some will often treat ivory, beige, tan, or light metallic gold as a neutral.

Be careful with metallics. There isn't enough light to make them work very well, on the lower sides of cars. In the past I've done artwork in pearl or metallic, and found out that on the lower sides, you usually can't see the metallics at all!