All metal water cooled tri colored single nozzle hot end for Delta Printers. He was able to get some horizontal color mixing happening to an extent. Three cubes printed next to each other each with a different pattern of vertical mixing. (bottom picture) The color change is not instant and takes a bit of extrusion before the other color is fully there. He said that he uses the infill for the color change over and that at 50% infill on a 30mm cube it took about 75% of the infill before the next color was available.
I found this via the Google+ 3D printing community posted by Herman Steyn
Hes also got a website with more photos and details about the project here http://deltaprinter.co.za/tri-colour/index.html