I just recently had to do a matchup with the 40 year old paint on my CB350F sidecover. I took it into the store with me and just went through holding the painted can covers next to the sidecover and guessing which one seemed to be closest. About the best you can do, really. But the good news is mine ended up working out just great! I found a couple colors which seemed to be about right, then found that can code in a touchup bottle by taking the color name/number from the can and finding it in touchup (I just needed a touchup bottle, but used the bigger and more accurate spraycan lid to match the color, then used that number to pick the touchup). When I got home I applied a dab of the one I thought was closest to the bottom of the sidecover (where nobody would see it) and let it dry. VERY close. So I used that bottle and took the second (unopened) one back.
You're best off bringing a part with you to the store and just eyeballing it. Especially true if some of the bike is going to stay the color it is now and you're trying to match that (the faded, old paint).
Another technique is to go to a mall or other area at a busy time and bring your part with you. Walk around the parking lots at the mall and see if you can find a car color that's super close. Then you can go to the auto paint store or internet and try to find a "newer Chevy HHR sort of wine colored metallic red" and find the actual paint code that way. Many automotive paint stores have HUGE color books with patches you can look at. And if you have a color by number they can make just a single spray can of it custom for you for probably $20 or so.