I'm not an expert but common sense tells me... Since you are suppose to focus on the front sight I would think it should be a more vivid color, with something maybe less vivid/bright for the rear. I don't know if this actually works but I'm sure someone here knows "how it should be."
At the same time, the advertisements for those gosh darn blue halogen headlights are completely wrong, blue is one of the worst colors in respect to "loosing your night vision" while red is the best. Why are chart rooms in ships lighted red at night? Gee I wonder... 100's of years of experience or some headlight commercial for something no one needs anyway... I guess I don't care too much for those lights. lol
All that said I would stay away from blue and consider red for the rear. But then again they aren't that bright anyway.
My USP came green on green, it looks nice and I'm not stalking around at night and keep a flashlight by my bed so I'm in no hurry to change them.
I know that red is the best color to use to preserve night vision, and yellow isn't too common in nature/buildings (at least not school bus yellow) and grabs your attention. (Off topic, school buses were once largely orange, being one of the most "un-natural colors" but they found that the color yellow was more "alarming" so they switched, according to the History Channel anyway.)
So my vote for the most affective combo would be yellow front red rear. Your all set unless you're in a field of daisies...