This is really good but achieving 10M colors with sharpie technique seems to be impossible. Do you think it can be improved further?
I mean the 10M value is a theoretical one, they're mixing base colors together to get more variations and the 10M value is basically linked to the dosing precisions of each color. I very much doubt that you could actually measure 10'000'000 different colors on their prints.
But your question in interesting, like would it be possible to make all of this more precise...
Without thinking too much about it I see two things to adress first :
1. Replace the sharpies with a precise ink-dispensing device sing a CMYK configuration (to start with)
2. Replace Ø1.75mmm filament with a smaller one, and make a custom hotent design with a very low melt volume to make it so that you don't need to purge too much to get quick variations of color.
The main issue would be how to keep the ink dispensing in sync with what get's out of the printer. I feel purging would still be necessary.
Another way to do it would be to design a filament that can be dyed easily and quickly and dye it after each layer.
You'd need an IDEX-style printer with one toolhead being an ink or dye-dispensing device akin to an inkjet printer, while the other one is a traditional FFF toolhed/extruder.
Process would be print one layer, park hotend, bring dye dispenser to locally apply all the colors necessary on the print, park the dye dispenser and apply the next layer of plastic with the hotend.