Reason for blue color of sky :
Tiny air molecules in our atmosphere cause white light to "scatter" as it travels through the atmosphere. As the wavelength of light falls, Rayleigh scattering—caused by these tiny air molecules—increases.
Red light has the longest wavelength, whereas violet and blue have the shortest. The sky appears blue during the day because blue light is scattered more than red light.
During sunrise and sunset, when the Sun is low in the sky, more light must pass through the atmosphere of Earth. Blue light is lost to scattering, so we can not see it, but red light is not lost as much, therefore the sky seems red.
Why sky look red during sunrise and sunset ?
It seems as though the sky changes colour during sunrise and dusk. We cannot see the blue light when the Sun is low in the sky because it must travel a greater distance through the atmosphere of Earth and is scattered away.
Since this light has not been very widely spread, we instead observe the red and orange light that is moving in our direction. As a result, during dawn and sunset, the Sun and skies appear redder.