If you really want the paint to be perfect, take in a piece of fabric the color that you want it to be....have the paint mixed and tested at the store before you pay for it. You will get a much truer color match than any of those little swatches in the store.
A piece of colored paper works too. When I was having the interior here painted before we moved in....I gave the painter a formal invitation that I had gotten in the mail and told him "I want this color...exactly". When it went on the wall and dried...it was absolutely perfect.
At my old house....I told the painter I wanted "chocolate brown" for the shutters. Apparently that was too ambiguous. He kept bringing me little test cans of paint and trying it, and they weren't right. I finally told him to go buy a Hershey candy bar and match the color. He did...and it worked!!
Color is one of those things that I obsess over....if it's supposed to match...then it better be exact....or it DOESN'T MATCH!! Drives M nuts...because I can tell if it doesn't.