The redness in atmosphere at Sunrise and Sunset is due to :

The red colour in the sky at sunset (and sunrise) is due to an effect called Rayleigh scattering. At sunrise or sunset, since the Sun is low on the horizon, the light rays must pass through more of the atmosphere – and therefore bounce off more molecules – than at other times of day. Thus, blue light gets scattered away before the light reaches your eyes. Other colors – such as red, orange and yellow –continue to pass through the atmosphere unaffected.