Jason Powell made his highly customized Rostock Max V2 delta with the Diamond hotend and upgraded the firmware to do color mixing. He made multi part .stl-s to enable multiple colors.
Detailed video:
Instructables project homepage:
http://www.instructables.com/id/Full-Color-Mixing-3D-Printer/
His Thingiverse page:
http://www.thingiverse.com/jjpowelly/designs