In case the sky appears red in one of its sides, this can be attributed to Rayleigh scattering.The sun comprises in it all the colours of a rain bow. The small molecule air and other particulates interact with it when the sunlight enters the Earth’s atmosphere. The wavelengths related to the blue color are shorter than those related to other colors. Thus, there is more scattering of the blue color. That’s the reason for the sky looking blue on a cloudless day.The sunlight passes through thicker part of atmosphere at sunrise and sunset as compared to noon. Most of the blue spectrum has already been broken down by the time the sunlight reaches our eyes, leaving longer wavelength spectrum colors like the reddish-orange-yellow end of light spectrum. Thus, why does the sky appear as if it was coated in red during sunrise or sunset?  

A lot of Rayleigh scattering is dependent on how much there is to look at in terms of atmosphere, sunlight has to move through. Blue light gets scattered away by the more of atmosphere traveled through by the light, thus resulting into more red color in the sky. This explains why most times, the sky shows some shades of redness in these occasions than it does midday.

Other factors that can affect the color of the sky at sunrise and sunset include:

The concentration of dust and pollution in the air. 

Dust and pollution particles can scatter light, thereby making the sky look redder.

The different types of clouds that are in the sky. 

There are a couple of things that clouds can do which includes: reflecting and scattering sunlight and making the sky colorful.

The position of the sun. As the sun descends in the sky, a longer distance have to be traversed by the light as it tries to reach us. 

More blue light gets scattered therefore making the sky appear redder.