Method and apparatus for printing toner images in black and at least two highlighting colors in a single pass of the imaging surface through the processing areas of the printing apparatus. Imaging and development techniques of color photography and tri-level xerography are combined to produce images...http://www.google.de/patents/US5155541?utm_source=gb-gplus-sharePatent US5155541 - Single pass digital printer with black, white and 2-color capability
