I dropped by the place that painted Pearl (a local auto body place) sometime last fall and they did that "Camera-on-the-paint" thing that Alan mentioned.
As we all suspected, even though she's "Pearl white", there are a bazillion versions of "Pearl White", all different, not to mention the color shade changing over time (like, how do you fade white?).
What the "Gonculator 5000" paint computer spit out for a mix formula was different for painting the car (two-part, base color, then metallic plus clear top coat) versus touch-up (single part with everything mixed).
I got a touch-up bottle to the new formula and it's matched perfectly - WAY better than the 20-year-old stuff I still had from when she was painted!