The way this is done (has been done) is printing with custom colors. Standard printing is done with CMYK (three colors and black). Purportedly high-end printers (I don't care if the printer is gold plated and diamond studded, if the driver / host-based preprocessor was written in 1999 using Visual BASIC then the printer is sh*t) have had an extra blue, or an extra red, for years.

In the back of my mind, I find it oddly surprising that we have 3D printers but I haven't seen a printer where you can mix your own pigments and print with them.

Many years ago I knew someone in Seattle who made microprints with burnished copper plates, some of them with a dozen different colors. He designed a couple of commemorative postage stamp-like objects, and that somehow got him hired by an eastern european country as an anti-counterfeiting advisor and then he disappeared.