The darker color in leafy vegetables are associated with antixoxidants, which are thoughts to have many health benefits.  A team of plant physiologists at the USDA facility in Beltsville, MD has used ultraviolet light-emitting diodes (LEDs) to make lettuce darker and redder, i.e. healthier.

This may allow for the opportunity to improve the quality of food grown in greenhouses, especially during the winter.  It may also be used to maintain the quality of food already in storage.

Link:

http://www.sciencedaily.com/releases/2009/05/090518172659.htm