As it has already been suggested, make the shop take the gas cap off and send it in to make sure the paint is spot on. There should be no excuses at that point, and the paint supplier will also be on the hook for mixing incorrectly twice.
For your inconvenience you should have them paint your amber reflectors to the same Mineral White FOR FREE. And if you wanted them painted, get them to paint your outer kidney grills and your fender grills too. Think of it like getting the iND cosmetic kit for free
(I believe its $350). Or maybe sucker them into doing those grill pieces ala Performance-style in gloss black.