I have a couple of race shops that have me print paint matched sheets for them to use if the cars get a bit banged up. This is the biggest headache job out there.
I always have a piece of the car to match, and I do it outdoors, where the car will be seen by most people. Once I get close to the right color, I start changing color values a tiny bit at a time and print about 5 4"x4" squares. If one looks close outdoors, I will then cut a V into the square and analyze the color right up next to the printed color. Once it looks right, I stand back a bit and see if it still looks right. If it passes both tests, I print away.
Oh yeah, Be SURE to laminate your print before comparing colors.
Chances are it will not look right under florescent lights, but it will outside.
Like I said, A huge headache job, but sometimes you can't design around it.
I do have one customer that took my printed color that he likes to his painter and had him match that. He is a smart customer that I will keep around for a while!