Why is the sky blue?

Why is the sky blue and not another colour?

The appearance of a blue sky or red one is related to the electromagentic radiation which we get from the sun, INcoming SOlar radioTION, insolation for short and the characteristics of light itself, when it enters our atmosphere. Inside the atmosphere, radiation can be absorbed, reflected or, most importantly refracted/scattered. It is this third process of refraction which gives the sky a blue color as a result of the particles in the atmosphere and the way they change the direction and wavelength of the incoming light according to the size of their own diameter. The two major types of scattering are Mie and Rayleigh and these cause blue and red skies repectively. Incidentally the ocean appears blue or green for the same reasons. Hope this is sufficient information to help you. Remember in Science one question often leads to a more detailed explanation you might not have expected.

or you could just say because the colour blue spreads more than the other colours in the spectrum

The sky is blue because light from the sun hits the earths atmosphere and is scattered a bit. blue is the colour most easily scattered, and shows up more.