system
Given the following post, respond as if you're commenting on Reddit:
human
What would the atmospheric composition have to be to scatter red light instead of blue? Green Light? Purple?
gpt
We actually have two examples - Mars and Earth - with different colored skies. The difference comes down to Rayleigh scattering vs. Mie scattering. 

There are two variables to keep in mind with scattering that can be wavelength dependent - the cross section and the scattered angles of the light. Rayleigh scattering is a good approximation for particles or molecules much smaller than the wavelength of light. Rayleigh scattering spreads the light at all angles, but the cross section is higher for blue light than red. This gives us blue skies and red sunsets. 

Mie scattering describes what happens when particles are about the same size as the wavelength of light. Mie scattering has a flatter wavelength dependence for the cross section, so it doesn't produce very vibrant colors. However, red light scatters broadly while the blue light scatters in a more limited cone. Mars has dust in its atmosphere that is the right size for Mie scattering. That makes the sky on Mars red and the sunsets blue. You can get the same effect on Earth if a volcano puts up ash that is the right size. This can give a visibly blue moon.