Well, the most comon technique I'm aware of if you google or go to pinrepair.com is the Acrylic paint method. This is the method I'm using on my Gorgar and have used in the past for others.
You'll need a decent Acrylic paint set (not the cheapest, but you don't need professional either). A color wheel is useful. It helps you by showing you what base paint colors to mix to get different types of shades (so if you need a darker green, it shows you what to mix, etc...). A decent selection of brushes (don't go cheapest, but you don't need pro). A mixing pallet (or whatever). A peice of plexi or small glass (you use this to match your color). Just mix a color to get close and paint a small layer on your plexi/glass and dry with a hair dryer. Then lay the glass over the area you want to paint to check for color match. Adjust your mix as needed.
This works pretty well and you can get a fairly close match pretty quick. If your touching up a playfield or cabinet, chances are you'll need a few color matches at the least. You'll go nuts trying to get someone to match them with a color match system. Besides, you can only get that paint by the can and that's way more than you need. Also, Acrylic paint is recommended because it is least likely to react with playfield coatings (cleaners, wax, clear coat).
Lastly, Acrylic is water based so if you mess up, you can just clean it off with Goof Off or some other cleaner like that and do it over. It's almost fool proof (not actually fool proof... there's always a better fool out there ).