Standard enamel as far as I know, since back then you got new plates every year instead of a decal to put on existing plates. If you can get the color mixed in IMRON or an epoxy based paint it will last a while.
We were lucky here. Vajenya used white on black background even years, and black on white background odd years. Easy restoration. NC used school bus yellow and black so theirs are pretty easy to restore too.
You can usually find NOS plates still in their wrapper for most states and years. The DMV's often sold them off to license plate dealers when they were cleaning house. I got most of my Virginia plates from a guy in Altavista who went by "Tag Man" at swap meets. He died in 1998 though.
If your state's plates are special colors, you'll need a good sample for the paint guy to mix it for you. Most good paint suppliers can mix you a spray bomb. Paint the background, then get a wide rubber roller to apply the othe rcolor to the raised areas.
"I don't know what the world may need but a V8 engine's a good start for me"
Cracker "Teen Angst", 1992