As well as a rather nice new website, Melbourne based studio, Famous Visual Services have a cool set of business cards, each with a slightly different colour shade across the back.
First they printed a solid fluoro yellow on all 250 cards. They then ran the cards back through the press adding dollops of fluoro red to the press as the cards were running. The result is a 250 step gradation from yellow to red across the print run. The name details were then letterpress printed in black.