Quoting evil
"Blue was initially considered a supplemental color, meaning that it made an effect to the light and coral color, but wasn't really necessary. That's changed though, and it really is a beneficial color that should be used in all arrays, although sparingly.
Blue LEDs do a great job of fluorescing reds and oranges that royal blues just can't touch. They also help to take some of the purple tint out of the light that you can get from a neutral/warm white and royal blue mixture, as well as making the light a little crisper. Too much though, and the tank can look like Windex, so they either need to be used with restraint, or put on a separate channel to be controlled independently."
