Alternate sky colors? Ok, so the sky is blue because of Rayleigh scattering - blue wavelengths of light scatter off atmospheric particles in the air (more/more easily than others) - I understand this pretty well.
So, if our atmosphere had a different chemical composition, would the sky be a different color (barring things like clouds and dust)? I assume that the size of nitrogen molecules happens to be the perfect size to scatter blue light, so what if we had 78% helium instead, or hydrogen, or gaseous zinc?
If this is correct - which elements would need to be predominant in the atmosphere to produce red, orange, yellow, green, indigo and/or violet skies? If not, what am I misunderstanding about Rayleigh scattering?
 A: Aside from scattering, an important contribution to sky color in a purely gaseous atmosphere is absorption. Already on the Earth we have stratospheric ozone which makes twilights blue (see Why is there a "blue hour" after the "golden hour"?).
So, if the atmosphere were made of (or contained a large enough amount of) chlorine, nitrogen dioxide, iodine, or any other colored gas, the sky colors would be quite different.
A: Rayleigh scattering results in scattering inversely proportional to the fourth power of the wavelength. So, no matter what the composition of the scatterer is, the blue will be scattered more than the red, as long as the main process is Rayleigh scattering.
If you want to have another color for the sky, this will need to be the result of a different process or processes. Or the light from the star may have a different spectrum, with blue light being very weak or missing.
An example of a non-blue atmospheric color is the orange haze in atmospheres rich in methane, like on Titan or on the early Earth. Rayleigh scattering has nothing to do with this color. It seems to be due to chemical reactions produced by UV in the atmospheric methane, resulting in hydrocarbons emitting some orangy light.
The red color of sky at sunset on Earth (or after volcanic eruptions) is due to a different type of scattering, Mie scattering. This happens when the scattering particles are much larger than a molecule, comparable to or larger than the wavelength.
