Find your nearest automotive paint supplier, some are at the individual automotive parts stores. The better suppliers will have a portable colour analyser, that can be used directly on the vehicle, to match the paint they can mix.

The replacement colour will be pretty much dead on, but the sheen will differ because of the age of the old paint. If you are touching up only a panel, try to have the paint break at a body line or door way. If the finishes are annoyingly different, a new clear coating can be applied to both the old and new paint, evening the finish out quite nicely.