Mosaic Manufacturing’s Palette 3 Pro is a filament splicer that allows you to print up to eight colors with your existing single-nozzle 3D printer.
It doesn’t require modifying your printer; instead you feed the differing filaments into the Palette first, and it splices them together into a single filament that then goes into the printer.
Watching it in action is pretty cool:
The Palette 3 Pro runs $800, and they also offer a smaller Palette 3 model that steps down to four colors, and sells for $600.