Sunlight looks white but contains every colour. As it travels through the atmosphere, it meets countless tiny gas molecules that scatter the light. Shorter wavelengths — blues and violets — scatter much more strongly than longer reds. This is called Rayleigh scattering.
That scattered blue light bounces around and arrives at your eyes from every direction, which is why the whole sky glows blue rather than just the area around the sun. Your eyes are also more sensitive to blue than violet, which nudges the colour we perceive.
At sunset the light has to pass through much more atmosphere to reach you. By then most of the blue has scattered away entirely, leaving the reds and oranges that paint the horizon.
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