Red light emitting diodes were easy, and have been around for a while. Blue LEDs were harder; Shuji Nakamura won the million euro Millenium Technology Prize for figuring them out. Green LEDs have been tougher still. To get a good white light you need to mix all three, so designers have relied on a work-around, firing blue LEDs at a phosphor that emits green light, much like the ultraviolet light from mercury vapour excites the phosphors in a fluorescent light.