alt Hacker News> Photo reproduction should be the target here...
There's an app called Hueforge that produces models that color mix to reproduce photos:
It's been around for years. There are databases of filaments with their TD values and color measurements to use. The blog really sells Prusa's attempts to do this with their own PLA, but there's a long history of color mixing in the community with extensive measurements of filaments that anyone interested should check out, too.
That looks/feels a lot like the Cerilica Truism approach (arbitrary mixing).
Will have to keep it in mind for when I get a printer which does nozzle/toolhead switching.