The objective of the E3D mixing extuder is to get fast color response with homogeneous output. Beside mixing various color filaments, it could also mix different types of polymer to achieve different print material properties.
Here is the last E3D extruder, the full metal Kraken:
http://diy3dprinting.blogspot.com/2013/11/e3d-kraken-full-metal-extruder-with.html
For a different type of custom DIY color mixing extruder check out RichRaps project:
http://diy3dprinting.blogspot.com/2013/02/diy-reprap-with-triple-extruder-setup.html
Here is the last E3D extruder, the full metal Kraken:
http://diy3dprinting.blogspot.com/2013/11/e3d-kraken-full-metal-extruder-with.html
For a different type of custom DIY color mixing extruder check out RichRaps project:
http://diy3dprinting.blogspot.com/2013/02/diy-reprap-with-triple-extruder-setup.html