While polishing some Stainless Steel PLA prints in a rock tumbler (more info on this from adafruit), I noticed a regular PLA print was also starting to develop a metallic finish. After a bit of experimentation, I was able to coat white PLA with a dark metallic finish.
The following picture shows what the print looked like before tumbling. Both Ollies below were printed in white PLA, with the same settings, one was run in a rock tumbler for about ~8 hours along with some yellow zinc coated deck screws.
The metallic coating is a bit fragile, it will come off if rubbed or washed, but it’s more durable than I expected. I’ve been using the Aquanet (purple) hairspray as a fixer to help preserve the surface and that has worked, but an acrylic spray or some other clear coat will probably be more durable and water resistant.