A lot of truth here, but I disagree a little.
I can match paint to 99% with tinting if necessary. But that's if its necessary. Most of the time we have variances that get us to where the color needs to be. Metal and plastic show paint different because of how they cure. As stated the way paint is sprayed, ambient temperature and panel temperature can all affect the way paint lays and cures. The amount of reducer can change it. There is a lot more that will change it too, but I could go on forever.
Blending clear is one of the easiest things I do almost every other day. If done correctly there is no visible detection after the fact.
You need to get ahold of Sevan, Wasco, or Ohio Auto Color. Probably in that order. They can do a prophet on the paint and get a match. A prophetic match uses a camera look at the paint from multiple angles, and comes up with a good match.