Why are there no green skies?

In short, its because the wavelengths your seeing are not "true colors".

In long form, your question is the definition of Rayleigh scattering.

My favorite explanation, one which was given to me in a high school chem course went as follows:

There are 2 ways your eyes perceive color, one is fairly basic; specific wavelengths of light reaching your eyeball, is one way. But, your eye can also see "color" by combining multiple wavelengths of light. (You can see blue as a wavelength, but your eye can also be bombarded by many different light wavelengths and your eye sends a message to your brain saying ????!!!!!!, your brain replies uh umm ...."blue"!

The earth has an atmosphere of particulates which allows certain wavelengths of light to enter, and reflects others. Lets think of the sun for a minute; its not really "yellow, orange, red" (etc). The true color of the sun is a neutral white, that is, the sun emits such a wide range of wavelengths that your eye sees it (though hopefully not directly) and says white. (It also says ouch, but that takes a while to become apparent :] ).

However, the earths atmosphere "scatters" light; and here's why:

As you stated (correctly); wavelengths are a major reason the sky has the "Blue" color it has. But many people don't fully understand what wavelength really is, and it actually determines the definition of "light".

On a more critical level, the wavelength of light is the distance in which the waves shape repeats (Usually written as the greek letter Lambda). The longer the wavelength, the shorter the frequency.

Now, the final and most important note about light is that it is, by definition the inability of absorption of a color that determines the light we see. (If you wear a red shirt, in reality, it means that your shirt absorbs all colors of light except red, which is then reflected to your eye).

So, with allllll of that stuff in mind; we can draw some conclusions:

As light enters our atmosphere from the sun, light of different wavelengths "bumps" into particulates that scatter the light across our sky. "Air" (the combination of the gasses in our atmosphere) has a tendency to scatter short wavelength light more then long wavelength light. Blue light is the shortest VISIBLE light in the color spectrum, and is thus scattered to a much greater rate then other colors in our atmosphere. When we then, look to up into the daytime sky, we see this blue light scattering and perceive it as such. As the sun goes lower into the atmosphere, the path of light from the sun gets longer (in terms of atmospheric distance). This causes all blue light, (and MOST GREEN LIGHT) to be scattered to an imperceptible level. Now, as to why you can't see green light during this process; in reality, there are theoretically times when you would be able to see green light in the sky. However, green light is not a highly emitted spectrum color from the sun, and is thus masked out behind the more prominent blue during the day, and red during the evening.

And thus, no green sky.

Good Question.