The most common types of white LED are made by having an LED which emits blue light, with a phosphor backing which turns blue light into yellow light; combined, the blue and yellow look white to the eye.
However, the quality of this "white" isn't great, since it only is two wavelengths of light. Better looking, but more expensive solutions use three colors in one LED... typically red, green and blue.
What if we used two (or more) different types of "white" light instead?
For example, a violet LED paired with a phosphor which emits green light will also look white, and will also produce poor color rendition (notably, it would poorly illuminate deep red colors), *but* if we have a lamp containing some blue/yellow leds, and some violet/green leds, then the *combined* light would produce much better rendition than either alone, and violet/green leds ought to be no costlier than blue/yellow ones.
A third type of white could be produced by a cyan led paired with a red phosphor. This wouldn't be great by itself (even though it might look white to the eye) because it would be even worse at illuminating natural violet colors than the blue/yellow leds are, but we if placed some of each of cyan/red, blue/yellow, violet/green LEDs into a single lamp, that lamp would have a nearly full spectrum, and very good color rendering.-- goldbb, Jan 22 2009 Ten years later, high-CRI white LED lamps are widely available, but they use one wavelength of excitement combined with a broad-spectrum phosphor mix. This is the same method as high-CRI fluorescent lamps.-- notexactly, Apr 23 2019 I can remember when blue LEDs first came into existence. Until then, it was just a thing that LEDs were green, red or yellow, and obviously you couldn't have a blue LED because LEDs didn't come in blue.
Then we bought a very expensive piece of equipment that had (for no particular reason) a blue LED indicator light. Made the whole thing worthwhile. In fact we may have bought it solely because it had the blue LED.-- MaxwellBuchanan, Apr 24 2019 random, halfbakery