Azuro

Meet Azuro, a tiny air molecule who bounces blue light all over the sky and lets red light slip past.

Sunlight contains every colour. When it hits molecules of nitrogen and oxygen, which are far smaller than a wavelength of light, the light's electric field shakes their electrons. The shaking electrons send light out in all directions. This is Rayleigh scattering, and it works far better for short wavelengths than long ones.

The scattered intensity goes as I ∝ 1/λ⁴. Blue light at 450 nm against red light at 700 nm gives (700/450)⁴ ≈ 5.9, so blue is scattered nearly six times as strongly.

That scattered blue light reaches our eyes from every part of the sky. Violet scatters even more, but sunlight has less of it and our eyes are less sensitive to it.

At sunset the light crosses much more air, so most of the blue is scattered away before it reaches you, leaving the oranges and reds.