Why does the sun change color during sunrise and sunset?
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As light travels through the air, it bounces off of molecules in the air. Since air molecules are small, they bounce shorter wavelengths (purples, blues, greens) more than longer wavelengths (yellows, oranges, reds). At midday, this makes the Sun slightly yellowish (because some of its blue light has been scattered away) and makes the sky blue (because you’re seeing the light that was scattered on its way to other points on the Earth). (Cloudy days, by contrast, are grey because the large droplets in clouds scatter all colors equally.)
Near sunset, the path light has to travel through the air lengthens. Almost all the blue light gets scattered away, so the Sun starts to appear a deep orange-red. And since most of the blue light gets scattered *before* it gets close to you, the remaining light that *does* get scattered shifts towards the red end of the spectrum, resulting in the rich oranges and reds of sunset.
After sunset, the light in the sky is only bouncing off of very high parts of the Earth’s atmosphere (since the Sun has set for the lower layers). Since the upper atmosphere is very thin, little light is scattered, and what little light is is bluish. That color is enhanced by the bluish color of oxygen and [the violet of ozone](https://en.wikipedia.org/wiki/Chappuis_absorption).
At sunrise, these processes play out in reverse.
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