If you haven't heard, LEDGroupBuy.com now has stock of the elusive 420nm True Violet LEDs.
True Violet is a spectrum needed for corals to grow properly that was absent since the LEDs start pooping up for the coral market.
The Photosynthetic Action Spectrum shows that at 420nm the activation of chlorophyll A is great. Most LED system you see as kits only come with the 455nm which is still an essential part of the action spectrum but lack the 420nm that is greatly needed. Combining the 420nm into your LED kit provides a great deal of light that is needed by corals for growth and overall health.
You can look at it this way, you can't give a human being only a protein diet and expect him to live healthy. You need to mix in all the food groups to get a natural healthy diet. This is the same for corals, they need a variety of wavelength to sustain growth and health.