Here's the problem.... A "scanned" paint sample is going to be way off. The only way a place like HD can come close is on a flat/matte finish. Their system doesn't account well for gloss finish. The coating is most likely powder coat anyway, not paint. That does not matter for touch-ups, however.
The idiot behind the counter obviously isn't matching by eye after using the machine formula. A true paint jobber can match by eye and be more accurate than the machine.
Go to a competent auto Paint store, not an auto parts store. Bring the end plate with you. Look under "auto body supplies" in your phone book/online for your area to find a good paint store. A good paint jobber can hook you up with the correct paint and teach you how to shoot it to get the textured finish (low pressure, large tip). Otherwise you might as well get some textured trunk paint and shoot it with some semi-gloss black on top. It would be so close you couldn't tell. I shoot a lot of paint in the summer (autobody), but don't have time to do a project like this for you (sorry).
Awesome amp by the way! Very jelly.