Question

What causes different colors to appear in the sky?

Answer 1

If you are talking about a rainbow it is refraction happening from light passing through precipitation. If you a talking about for example a sunset, it is the sun's light being displaced as the Earth orbits it. The normal color that should show after the sun's light passes through the atmosphere is blue.

Answer 2

Molecules and small particles in the atmosphere change the direction of light rays, causing them to scatter. Scattering affects the color of light coming from the sky, but the details are determined by the wavelength of the light and the size of the particle.