The orange/yellow (it's common to put them on the rear with green front) is not dimmer but the green stands out more so it appears more vibrant. I read something about green being a better color to catch your eye than other colors.
cmdrdredd is correct, in night attuned eyes green seems brighter. Part of the reason is that Green is in the middle of spectrum and your red, green, and blue receptors will pick this up in low light levels. Light further to either end of the spectrum will be sensed by fewer types of color receptors. There is an excellent long and technical article on this at:
My preference is to have 2 yellow dots on the rear sights and a green dot on the front sight. In some night training, it isn't uncommon for the front sight to "disappear" when in reality your mind is confusing a rear dot for the front dot. As I get older I keep wanting a bigger brighter front green dot.
Some prefer a single tritium dot on the front, but front and rear helps precision. I try to envision a situation where the BG is lit enough for positive ID, and the defender is in the dark. My old master eye must now defer to my non dominant eye for this kind of sighting. The left hand is now being used more often for sighted fire, but the good old righty still works for point shooting in a hurry.
Love the Meprolights! Green has always been good to me. Last year, I thought about the orange/green setup, but thinking to myself that it won't actually make a difference to me. Either way you go, the whole set should run you the same price...mismatched colors or all green.
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