Oh, ha ha, should've thought that through! My bad. Well when I first read it, I did think it sounded a little strange. But they should'nt cancel each other out, but I'm not sure they'll compliment each other either... the U.V will MULTIPLY all the colours by Ultra-Blue, so colours with none aren't affected, and blue and white are really affected. The cathode will ADD red to everything, so everything will be a bit red, but the really-blue blues will become purple, and the ultra-white whites will become pinkish.
Or at least, that's as far as I can surmise in my own head from the little knowledge I have of the electronic spectrum. To be honest, a red cathode is only a couple quid, and I doubt the U.V is much more. If you've already spent £500+ on a rig, who cares about an extra tenner? You should just try it out! (And maybe post the pics up here for future reference, and my own devilish curiosity!)