Red Algae Pigment, predigested Plankton and Krill are examples of color enhancers. Carotenoid is another color enhancer.
Some people believe that color enhancers are artificial and are harmful. They are not. Fish get these color enhancers in the natural environment.
Related Links
http://www.cichlid-forum.com/articles/color_control.php
http://www.sonnysfishroom.com/?page_id=252