A clear overhead sky is a medium blue.
Ignoring the effects of solid particulates, what color would the sky be if the atmosphere was 100% Nitrogen?
And 100% Oxygen?
And some change in the ratio between Oxygen and Nitrogen?
Could a sky hue change be expected if there was more Xenon, Neon or Carbon Dioxide in the air?
What would it take for Earth to have a green or indigo sky that still supported life as we know it?