That is why I asked y'all about the "stand-out" features of Skylines. Also, I know it will be a decal on various background. Probably a transparent base with maximum of 2 colours will be nice, simple and easily spotted. You lots have to think about the "shape" of the base too. Most stickers I saw for clubs (car enthusiast) are rectangular - horizontal. I think vertical base is nice.
Also, you need to have at least 3 colour tonnes (designs) available for various background. Cars come in dark, medium and light colour. Sometimes, the colour of the car won't look good with either white or black stencils.
So it's up to y'all to see through this matter. Of course, you can forsake such details and opt for the easier feat.