I had bumper, rear deck lid and skirts painted at a local shop that does not specialize in high end cars and it being a 2000 Boxster they were able to match it spot on.
As was said before there are about three variations of the paint color, but even taking that into account they can spray a test piece of material and let it dry to see how it matches prior to painting the whole bumper.
If they could not match or mix paint right the first time I would try to get a refund and go elsewhere, any paint shop that is worth a thing can match up paint regardless if it is years old.
Good luck
