I'm assuming they aren't gonna go by the paint code because of a few possible reasons....
The car had a full repaint already and the current color code is not known or the car was painted the factory color but the color shade is off a bit? If the paint is original, then the paint code should be all they need to mix your paint. Also let it be known, if the car is a certain shade of color, especially silver, the paint may not match well if you just paint the bumper and do not blend the paint properly.
As for removing the gas cap door, it's gonna come off a few ways.. Bolts behind it or the swing hindge is welded and it won't come off.