These colors may look familiar to those who spend a lot of time on eBay, if nothing else. Skimming the surface of molten bismuth: https://www.instagram.com/p/BbG5EyBHcBI/
This is a pot of molten bismuth metal. It oxidizes on contact with air. When you scrape the crust off the top, the newly-exposed metal starts to oxidize, and goes through the color progression that we see on coins -- only it does it in seconds, not years. This isn't someone doing artificial toning. This is just a demonstration of a different metal toning the same colors as silver, very quickly.
Bismuth has a low melting so it's easy to grow some pretty cool crystals. As @-jeffB said, the colors are due to varying thicknesses of the toning (Bismuth Oxide in this case) that forms on the surface. Better living through Chemistry
People have been doing it to coins in hours for a good many years. And doing it in such a way that it is indistinguishable from natural toning.
