Why is the sky blue?
Air molecules scatter sunlight's short blue wavelengths far more than red ones, so blue light reaches your eyes from every part of the sky.
Going deeper
This is called Rayleigh scattering. Shorter wavelengths scatter far more strongly, so as sunlight passes through the atmosphere, blue light gets bounced around in every direction.
Violet scatters even more than blue, but sunlight contains less of it and our eyes are less sensitive to it, so we see blue. At sunset, light travels through much more air, most of the blue is scattered away, and the reds and oranges are what's left.