Word on the street is that the percentage of oxygen in Earth’s atmosphere has varied widely over time-- up to 32-35% during the Permian, and at other times down to 12-13%. Would this have affected the sky’s appearance in any way, by altering the color or perhaps by making a difference in the process of cloud formation?
I think that Rayleigh scattering is dependent only on the size of the molecule - as long as the atmosphere were mostly O[sub]2[/sub]/N[sub]2[/sub], it would appear blue looking up and red looking into the Sun. Now, if the atmosphere were 80% nitrogen, 10% oxygen, and 10% CO[sub]2[/sub], I think the color would change since CO[sub]2[/sub] is a much bigger molecule.
Cloud formation would also be unaffected by changes as long as O[sub]2[/sub] and N[sub]2[/sub] dominated - the adiabatic lapse rate depends only on C[sub]p[/sub], which is the same for all diatomic molecules. Again, if there were a significant proportion of monoatomic (e.g. Ar) or triatomic molecules (e.g. H[sub]2[/sub]O, CO[sub]2[/sub]), this might change significantly.