This is an excellent video show case from RichRap, on how to mix and blend colour for plastic filament or different materials with a RepRap 3D printer.
The set up is 3 individually connected extruders forcing filament into a single hot end, which then blends and pushes out the final filament.
Have a look at part 1 for an introduction and part 2 to see how the setup was actually achieved.
Part 1
Part 2
