3D, on re-reading your post I can't help wondering if you are confusing night vision with night vision goggles. I am sure you know that the eye has 2 different types of cells in the retina - cones which are responsible for colour vision and are packed in to the fovea (the central area where light is focussed) and rods which cannot discriminate colour and cover the rest of the retina but are not packed as closely as the cones. When light levels drop (ie it gets dark) the iris opens to let as much light in as possible and the rods undergo a chemical transformation which renders them very sensitive to light. The cones are just not sensitive enough to work at low light levels so the area you usually use for vision - the fovea - is not functioning (this is why you are taught to look slightly away from an object in the dark to be able to see it - the reflected light from the object must fall on the rods if the brain is to notice it.
Clearly this limitation on eyesight is not much use when you are staring down a pair of toilet roll tubes (NVG) which is why you use the cones which receive high levels of green light (around 550 microns wavelength) from the tubes. So although you are flying around in the dark, you are using the eyes' daylight vision (photopic) instead of night vision (scotopic). Coming out from under goggles (ie failure of tube, battery etc) is not usually a drama as the low levels of cockpit lighting required to allow the NVG to operate efficiently, allow your rods to undergo their normal adaptation to darkness. In effect you get the best of both worlds as you have day and night vision at the same time.
I believe another reason for using the green image is that it reuires less power to produce a certain brightness using green/yellow light than in any other colour.