The sky is blue because of a spectrum of colours. These colours come from the Sun, which is made out of Hydrogen and Helium. When light travels at a speed at 8 min/AU, It reaches earth. The structure of earth defines the fact that the sky is blue. Raindrops act like tiny prisms; They make a rainbow. This makes Blue wavelengths drop down, as science proves. But white wavelengths come right down at you, and other ones are somewhere in the sky. But blue deeps the furthest, hence its sky presence.