The sky, which is made up of gas molecules, is blue because of the random scattering of sunlight by the molecules. Rayleigh scattering defines the amount of scattering of light rays. Blue light scatters much more than red, which is why the sky appears blue on a clear day. Depending on the time of day, the sky may appear different colors.
At dawn or dusk the sky may appear red, orange, or even green and purple depending on how low the sun is and how close it is to night.
