Question

when sun set why the colour of sky is ret

Answer 1

when sun set the color of the sun will be red because when sun sets the sun is far from the horizon and the sun light have to travels longer distance so that the red color having longer wavelength will reach the observer and other colors get dispersed away from the observers eye 

Answer 2

The red colour in the sky at sunset (and sunrise) is due to an effect called Rayleigh scattering. There is a similar form of scattering called Mie scattering which is responsible for the white colour of clouds.