Carl,
I don't know a thing about painting, but if I were attempting what you are doing, I think I would get a fairly large piece of material that I could paint at the same time I was painting the car, using the same primer, base coat, etc. Then when you're ready to do the stripe(s), you would have a good platform to experiment on for getting the desired color for the stripe(s) before actually painting them on the car.
Just my two cents.
Ted