When I moved to Utah back in college, I met a guy who used his old plates as, well, you wouldn’t call it art — although his father was a painter and had started this family habit — but as ornament. Their capacious garage, where they repaired old cars and especially Ford trucks, was lined with old Utah plates. My buddy even had a couple on the inside of his 1-ton Dodge flatbed truck, most of which was covered with bumper stickers. Bumper stickers on the inside.
Utah plates back then alternated from black on white to white on black, very functionalist, like everything in the state. This was before they got into “Ski Utah!” and then the redrock Delicate Arch logos — in other words, it was before Utah californicated itself.
California itself actually resisted the self-promotion trend longer. For a long time, California had yellow on black plates, the shade of which I believe is the source of the phrase “California yellow” when describing cars. Then it shifted to that egg-yoke yellow on bright blue plate. Both of these are now collector’s items, and if you see one on a car you are witnessing one of the most precious of automotive status symbols. An old yellow-on-black plate has at least as much motoring cache as a Lexus and probably as much as a Porsche with the current blue-on-white with script “California” plates. An original VW with yellow-on-black plates screams CRED. You could probably trade it for a Bentley, but no one would.
Nowadays vintage plates are bought and sold on eBay. You can’t use them, but they make great bar decorations.
This home-owner in the Chino Canyon area of Palm Springs, where there are lots of noteworthy Modernist houses designed by Richard Neutra, John Lautner, Donald Wexler, has made his statement out at the curb. A mailbox made out of plates (more likely a mailbox wrapped in plates).