I’d suggest getting some raw steel and priming and practicing on it. It’ll be a good learning experience, and if the color doesn’t match perfectly you’re better to become aware of it on a test piece of metal.
There are stores that sell proper automotive paint rather than the rattle can stuff, and it does a much better job (with a bit of practice), especially the clearcoat. The place in Nashua NH will even put it in a pressurized container for you if you don’t have a compressor. Since you’re only doing a gas door, you should have plenty to practice with. It’ll be much more costly, but does a far better job. You’d have to seek such a place out in your area. Perhaps a body shop can refer you to one.
NOTE: also check the parts store for the proper rubbing and polishing compounds. “Polishing” is generically used for anything that creates shine, but the compound I’m referring to is actually a abrasive compound that come sin various “micron” sizes (the abrasive sizes). Personally, I like to do the final polishing steps using a wet-sponge polisher pad.
I’d suggest also getting a book on automotive painting at the local bookstore, reading it first, and using the gas door as a learning experience. It’s a perfect minimal-risk opportunity to learn automotive painting, and that’s a skill that will pay dividend forever. A well done paint job is a source of pride, and justifiably so. Most people never have to opportunity to lean to do it well. To me this sounds like the perfect opportunity.