if ur trying to blend old paint with new paint, it's very difficult. it's your repainting the car then most reputable places do a good job of getting it matched
Most places that mix automotive paint use a hand held scanner that can 'read' the existing paint and provide a formula to match the color as closely as possible. This is very usefull when trying to match a color of paint when a paint code is not available or unknown. You would need to provide a flat piece of painted metal for the scanner to get an accurate reading. This process is not always 100% accurate with old or faded paint. Another thing to consider is the older metallic paints sometimes used a different metallic flake than what is usd today which can affect the paint color.
yes most can do a resonable job as long as the got a "prophet" it reads the paint pigments,here is a job i done a while back (just had the paint mixed and sprayed owner done the fitting or lack of so far.this was matched after i took a 30 yr old sample that i rubbed out by hand to get some depth back. It does match good,not perfect but I do believe when the rest of the ca is buffed and cleaned it will be difficult to tell it is not the same base paint. http://twitpic.com/qxaii/full
