I've never implemented one myself, but automated spray-painting machines are pretty common in industry: https://www.durr.com/en/products/pai...paint-machines Usually they're mounted to robot arms, but I suppose you could put the same hardware on a Cartesian machine. As long as whatever you're painting is basically flat, all you'd need is a sprayhead for each color, and a way to turn each color on and off once it's in position.