When the sun sets, its light isn't just grazing the Earth, as seen from where you are, but it's also coming through a lot of Earth's atmosphere. That means a lot of molecules and small particles in the air, which change the direction of light rays. Light is made up of lots of different wavelengths, which is why we see color. Bluer light more easily bounces off molecules in the air, while redder light doesn't. That's because bluer light has shorter wavelengths while redder light has longer wavelengths.
