Why does the sky change color?
Basically what the title says. What mechanisms are significant and how do they contribute to make the sky blue. Also when the sky is not blue, like when the sun sets, how does it happen?
Because of scattering. Blue light scatters more then other colours, because of its high frequency.
For more information read
Sunset & Sunrise sky is reddish coz red light with longer wavelength tend to travel longer in atmosphere.
And when Sun is up in the sky Blue light is reflected mostly by the gases in atmosphere. Visible lights with longer wavelengths pass right through(actually, around).