Generally speaking, it is an effect performed by painting in a subtle color of the vehicle color itself. This is easy to do when painting, but with vinyls, being a 'Show Truck'.... your client is cheating, but toady..... who isn't ??
If you have a red vehicle, go to either side of his red and that could be your color. Like a sightly orange-colored red or a slightly purplish red or marronie color. Knowing the color wheel will definitely help you in your quest for the right color.
Anyway, it's supposed to just slightly show up and using old flip-flop paint was a really cool way of doing it back in the 70's. I spray painted quite a few back in the day and this created an even neater look. It only came out on certain angles or lighting. We used to hand stripe metallic cars with the same color as the body and leave out the metallic effects and that really looked cool, too. We always called it and understated elegance.... and that's just what it was.