Why blue and not red or some other color? Because blue is more easily absorbed and scattered. Although the light coming from the sun looks white, it’s really made up of all the colors of the rainbow. Each of those colors has a unique wavelength. Red light travels in long, slow waves and blue light travels in short, quick ones. So because blue light travels more easily and because molecules in the air scatter blue light from the sun more than any other color, we have a blue sky.
[Interesting fact: Look at a picture of a rainbow. The blue is always on the shorter side (the inside) and the red, on the longer side (the top).]