Paint does not match

On my last project, I used NAPA base and HOK show clear. I took the gas cap door and the paint code with me to the store. After getting home with my new paint, I discovered it was too dark. How can I confirm the color of the paint is correct before I leave the store? I thought about using an airbursh gun to spray a test card and then hit it with some spray can clear before leaving but there has to be a better way!