This looks great, slicer software may be harder to create, but possible.
I have small idea to use any changing color for infill or support.
For example it takes X cm to fully change color, use that X length on infill or support materials.
@Mateusz_Perlak I think that it really depends on how you handle the extruder driver in firmware. If you handle it well in FW (it would be a pain) you could simply send a color string. assume an empty string was either w or k. but if you handle it all in the slicer it will be crazy.
@John_Ruddy We talked about this a few days ago!
@Sanjay_Mortimer This is exciting! The E3D hotend was a great upgrade to my printer, and I’m looking forward to seeing this in the near future! I’m volunteering myself to be an early adopter/alpha tester 
@Camerin_hahn I like the idea of handling this in the firmware, with RGB/CMYK values sent in from the slicer (as an additional variable to the E axis G-code). Presumably, if you calibrate the extrusion rates properly for each color, you should get a fast enough color change (on the order of one or two layers judging from the video). Otherwise, an idea might be to have a wipe tower off to the side (for example, a bracket attached to the X-axis on the Prusa i3). For more precise color changes, the extruder would near the wipe tower, extrude enough material to get the proper color mix, then wipe off the nozzle ooze against the wipe tower. (Could also have a servo-arm, that would swing across the nozzle to pick up the excess material)
@B_K11 you can also use some kind of wire brush, extrude into air on one side and travel through it to wipe clean. E.g http://www.thingiverse.com/thing:3123
@Sanjay_Mortimer Fantastic! - that is looking smooth and well mixed - and fast! Did you use active mixing or passive - or active-passive.
I used an active-passive rotating PTFE bar in my most recent dual mixer on 3DR, but the mixing you have looks even better, nice job @Joshua_Rowley
Can’t wait to see more of this project. everything out of one nozzle really helps keep up the quality of prints. That’s the one major down-side of multi-nozzle systems at the moment.
Keep the updates ‘flowing’ !
@Sanjay_Mortimer , most excellent work.
Out of curiosity, how are you handling varying melt temperature of different color filaments?
It’d be great to have another option for material switching than dumping in infill, because we might want to have solid infill with controlled material properties. I like the idea of extruding a bit somewhere else and scraping off the excess before returning to the print, like an inkjet print head clean.
Also, is colour change faster with a bit of selective retraction?
I think “unstable” material color may be dumped into fillings or support. However as mentioned before, it would require slicer to do the trick, not printer firmware.
How is it going with development of color mixing? any news, video or anything?
We’re waiting on the next prototype to come in, we’ve most of the parts, just missing one precision turned tiny part that we can’t make ourselves. The next prototype should actually fit on a printer too!
More updates Sanjay - the suspenders are killing me!
To say this Mix’struder intrigues me, would be an understatement…
Soooooon!
Sooooonerer!