In practice, its not quite that simple. The ones I have it seems the red are much more efficient at picking up light than are the green, which makes red better in lower light, but in bright sunlight the red are too bright and glare. The green aren't so much help on a dim day but are great in the bright sunlight.
At least with the sights I have the differences in efficiency of the light pipe pickup outweights the fact that normal eyes see green a little better than red.
Next time I break one, (so far I've only broke one and didn't notice it until cleaning the gun, I replaced the red with green) I'll try a yellow to see if its a good compromise.
--wally.