I'm really surprised no one has tried out mixing some in. From what I've read and heard on other forums, turquoise/cyan and deep red create a 'white' light (that brings out reds/greens).
I can't believe more of you don't think your tank looks 'washed out'. When I tried out a fixture I made for a reef that had a mix of cool white, neutral white, royal blue, and blue (all individually dimmable) I noticed that the green and especially the red spectrums were really lacking for me.
Violet LEDs are also really supposed to help warm up your tank.