Why the sky is blue

Sunlight looks white but contains every colour. Air molecules are far smaller than a wavelength of light, and tiny particles scatter short wavelengths much more strongly than long ones.

Blue and violet get bounced around the sky in every direction, so wherever you look you see scattered blue. Your eyes are also less sensitive to violet, which is why the result reads blue rather than purple.

At sunset the light travels through far more atmosphere, the blue is scattered away entirely, and what reaches you is the leftover red and orange.