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After covering myself with ink (and completely destroying the Red sharpie) I discovered you can carefully take apart a sharpie w/ a pair of pliers. You can then remove the plastic wrapped ink reservoir, cut it open with a sharp blade and wrap it around the filament before it gets pulled into the ez-struder.

I secured it w/ duck tape which let me put it on and pull it off in various places - Multicolor! Easy, inexpensive, but a bit messy.

See the pic, colors used fuscia and baby pink on glow-in-the-dark abs.

Looks nice. There is a way to do it without destroying the sharpie, just print out a design that passes the filament through while putting it in contact with the tip of a sharpie. There is one on thingiverse that does that.

If you have three separate sharpies controlled by solenoids (PWM capable) you could vary the amount of ink of different colors being put onto the plastic. Real full color printing with that; you would only need the solenoids, a printed part to mount stuff, and a lot of code modification to change the heater cartridge outputs of the board to color ouputs.

Everything is in place easily except the code and slicing stuff. Too bad there are several overly broad patents that prevent commercial use of this kind of stuff!