Is the color of the faint natural airglow green?
Yes. Usually you see the 5577A line emission as green as seen in the Gordon Garradd photos mentioned above. What color are the Gegenschein and Zodiacal light? They are simply scattered Sunlight, so they are exactly the color of the Sun. (The Zodical Light dust particles are “big” so they don’t scatter blue preferentially, unlike the small particles of air, which are roughly the same size as the wavelength of light.) I’ve been able to convince myself that the very brightest part of the zodiacal light near the horizon, probably close to the time of astronomical twilight, is right at the threshold of color for me and is a funny turquoise blue-green, i.e. right at the peak dark-adapted color. If you’ve ever looked at a discharge tube of oxygen with a simple cheapo replica grating and seen that gorgeous line at 5007A, that’s the color, but barely perceived. Chris Schur told me once he sees the color very strongly, but for me it’s a marginal thing. Could the dark night sky background be seen
