You guys are on the right track. The human eye is most sensitive to 555nm light, which in the color spectrum falls in the realm of green. This is during "photopic" or daylight conditions, where the cones of the eyes handle most of the eye's vision.
During "scotopic" or low-light situations, the eye is actually most sensitive to lower wavelength light around 500nm which is a blue-green. At this light level, the rods of the eyes typically handle most of the vision and are relatively incapable of seeing higher wavelength (such as red) light. However, red light can be visible at certain brightness intensities. The advantage of using red light is that it does not affect your night adaptation. Hence the reason why some lights are red for .mil when intended for night-time use.
I'm not sure if this is the reason why red-dots are indeed red, but it makes sense to me, anyway!
"We are oft to blame in this / 'Tis too much proved -- that with devotion's visage / And pious action we do sugar o'er / The devil himself."
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