Ultrastable evolved into Evercolor, which had their CMYK coatings made in high volume by an Agfa plant. Charles Berger was involved in both ventures. These, as well as Polaroid Permanent, were all screen processes. If the dot density got too fine, the layers would blister. The halftone pattern was evident, and to many, including me, the look was too much like halftone offset printing. I visited Evercolor's operation, and still have a some samples stored away somewhere. The colors could also block up an get garish; but others in the know have informed me that was due to post-scanning software issues.
In these carbon processes, you don't have the ability to fine-tune the outcome during the printing process itself, like with dye transfer printing. You also don't get the transparency of dyes. But
certain local hand-coaters used relatively transparent watercolor pigments and got lovely results. The amount of time and effort they put into each print was ridiculous. Gum printing is based on painting on multiple layers instead, and can be altered somewhat, but can otherwise be a muddy-looking unsharp result. Some apply gum coloration in register over black and white silver or Pt/Pd prints - more an artsy thing than a realistic look. Depends on your taste.
Bill - the in initial hurdle is in the separations. They can be sequentially done in camera using long-scale panchromatic films (large sheet film is far easier to register together than small film). There were also bulky tricolor cameras using beam-splitters or pellicles made by Devin and Curtis up through the 1950's. Some people have reconditioned and upgraded these at considerable effort and use them today, though they're a lot of effort even to load - and you get only one image per loading. Then chrome sheet film became the dominant mode of shooting, with the color separations being my in a darkroom or consigned to a commercial service bureau specializing in that. Today, it's more common to make scans of a chrome or sometimes a color neg and output that to some kind of set of RGB separations using image setting film, laser film recorders, or transparent inkjet material. They are all kinds of potential hybrid routes.
But color carbon printing is daunting enough that few people get beyond monochrome carbon printing, which is actually fairly popular. Most of these UV processes involve hazardous dichromate sensitizing salts; so there are potentially serious health risks if one is careless with such chemicals.
Of course, one could simply make in camera-separations, or even lab separations from chrome film or slides, and then enlarge those separation sequentially in register onto ordinary RA4 printing papers. I have the ability to do that if I wished, but the needed equipment and degree of dust control and enlarging precision each step can be daunting, and the equipment itself quite expensive if you don't already have it like I do. High quality results often require supplementary masking, so maybe a dozen or more sheets of film get involved, and the cost adds up fast.
But sure, Bill, there are plenty reports of people just casually fooling around and having fun with RGB filters, taking a triple exposure, mainly with DLSR's, then reassembling the results using a PS snap alignment feature. Fine for web purposes and a fun funky look; maybe not so worthy of being framed on a wall. Real printmaking is a whole different skill set; and there's nothing easy about it.
But when well done, it will sure send all your webbish little pixel pixies scurrying back into their burrow with shame.
And as far as taking a single black & white shot and colorizing it - might as well use Marshall's oil colors, or now some Ai program, whatever ... I can't personally conceive of that as real color photography at all, though some people enjoy doing that kind of thing.