Light acts as a flow of particles called photons, but it also acts as a wave. And it is the wave nature of light that determines colour.

Red is low frequency (it has a long wavelength, the distance between peaks)-

While blue is high frequency (it has a short wavelength, the distance between peaks)-

Long wavelength powers through air  without scattering much, but short wavelength gets bounced around by the impact against air molecules and gets scattered. (image credit Getty Images).

So, we see more of the (blue) light that’s caught up in the air than the (red) which passes straight by us.

Sunsets tend to be red for the same reason. The blue light has been scattered on its way through the atmosphere and we’re left with the red that was able to reach us.

Incidentally, in space, there’s no air, so no light is scattered and the sky looks black.