This isn’t rocket science and it’s certainly not original, but I finally screwed up enough courage to start routinely swapping in a new filament color without pulling out the old one. The trick is to cut both ends flush (with a flush-cutting wire cutter) and maintain gentle pressure on the new filament so it slides right into the grip of the extruder drive gear.
Seeing as how I need tchotchkes in a big way, I run off a plate of Chalk People whenever it’s time for a new color:
The transition between yellow and black was rather weird. Fortunately, the gory details remain hidden inside that quartet of Chalk Women.
These have all the right attributes for a tchotchke: fast printing, not much plastic, smooth edges, a little fill to show how it works, a few small defects for education.
The Smell of Molten Projects in the Morning 
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2 responses to “Thing-O-Matic: Bicolored Prints”
Pretty neat!
And, relevant to the Extruder Zit question, the bottom filament pops up out of the extruder when the top filament retracts. I can see about 5 mm of the filament below the extruder drive gear if I stick my nose into the box at the right angle.
The filament retraction distance is about 3.5 mm (25 rpm / 60 s/min * 0.125 s * 2pi * 11 mm), so there’s enough pressure to drive the filament more than one diameter back out of the extruder! Some of that is due to molten plastic pressure, some to filament mechanical compression / bending in the PTFE tube, and probably some from sheer perversity, but it goes a long way toward explaining where those end-of-thread zits come from.