a tube with two inputs, (or however many materials you want) and one output. one extruder, a snipper on the ‘V’ output, and two stepper motors.
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one stepper motor, a switch that moves a gear to the roller that feeds the appropriate fillament.
the switch is probably a solenoid,
servo
the main stepper motor could stay, that grips the fill after the cutter. two small motors, one on each branch need only turn on to start the feed to the main motor.
no idea how to wire the motors so that repetier says when to switch filaments.
two stepper motors, one extruder, ; maybe the program can be controlled to retract the filament instead of cutting it…
You should check out this Kickstarter project. : https://www.kickstarter.com/projects/mosaic3d/the-palette-3d-printing-evolved/description 1
They have something cool developed already 
Cheers
10 days later
Thanks for reties, I’m thinking the color palette is good. Curious how it times what color to advanced, when the different pigment is on the same layer.