I've had very good results by properly using good quality plastic and vinyl paint on good quality plastic parts, appropriatel prepared, see here and here for examples.
I've had paint mixed using the color scanner and most of the time it works very well. As you mentioned, a knowledgeable paint tech will know which additives to use in a plastic and vinyl formula.
For solid colors I've also used the color spectrum chart which has chips that gradually fade from one color to another. Many times you can get a nice match that way.