Exists. Raw links:
http://www.richrap.blogspot.co.uk/2012/08/3-way-quick-fit-extruder-and-colour.html
http://blog.reprap.org/2012/09/colour-mix-progress.html
http://www.reprap.org/wiki/File:RepRapColourMixingReport-jmc.pdf (pdf!)
http://forums.reprap.org/read.php?1,159728
http://www.google.com/patents/US20100327479
