The following is what I gleaned and may not be entirely correct.
The way to retain night vision is to ensure that the pupils don't contract, and the light-receiving cells on the retina are not exhausted from exposure to increased levels of light.
There are 4 types of cells on the retina, 3 of which respond to the colours red, green and blue. The fourth responds to the light level (brightness or darkness). I believe this particular cell is shaped like a rod.
While light levels are very low, vision is perceived in black and white. All the light-receptive cells work in unison to deliver signals to the brain, the rod cell being the most responsive at this level. But as light levels increase, the individual cells begin to deliver signals specific to their colour receptivity. When the cells deliver signals for higher levels of light, it takes a while before they recover and are capable of delivering signals at lower levels of light.
Hence to retain night vision, the lighting level must be kept low in general and the light should be of 1 pure colour. This is so that the only cells in the eye that would be activated at the higher levels would be the ones that respond to that particular colour. All the other cells would still be receptive of lower levels of light.
Also, since the eye is more reponsive to green, it is argued that there are more cells in the eye capable of detecting green, though it is highly possible that the cells that respond to green are just capable of delivering a stronger signal. So some people use a light that will not affect the green receivers, which is red. Others suggest that since the green cells respond the strongest, you should use a dim green light instead, since very little light is needed for a response, thus not just saving the other colour receiving cells but also the rods.
Personal experience, and seeing all the military night-vision devices using green suggests that a dim green might be better. But not bright cyan.
