"I have a guy I work with who has beached about the paint not matching when he has sprayed a spray out card that DID match. What do you say to a guy like this? I calmly asked, "How could the spray out card match and the paint on the car doesn't"? SOMETHING changed between the spray out card and the car, SOMETHING changed. He honestly believed it was the paint that somehow changed, I am not making this up! "
Did he wait any length of time between the spray-out card and spraying the panel. Bet you dollars-to-doughnuts it settled out in the gun, OOPS. Especially with whites, the toners all have a different specific weights to them, mix the paint ready to spray and leave it sit in a mixing cup for 15 min. and see if it settles with the toners on top. With a gravity feed gun you'll be spraying pure white toner first, with a siphon feed gun you'll be spraying color toners first. With pressure feed you have to deal with the length of the hose.
With color matching EVERYTHING matters.