Why are sunsets red and orange?

Short answer: sunsets look red and orange because sunlight near the horizon travels through much more atmosphere than sunlight from high overhead. Along that long path, shorter visible wavelengths—especially blue—are scattered away from the direct beam more efficiently than longer red wavelengths.

The same scattering also makes the daytime sky blue

For particles much smaller than the wavelength of visible light, Rayleigh scattering becomes much stronger at shorter wavelengths. In the daytime, scattered blue light arrives from all across the sky. Near sunset, the direct beam has crossed so much air that much of that blue light has already been redirected elsewhere before the remaining sunlight reaches you.

Aerosols can change the show

Dust, smoke, pollution, sea salt and volcanic particles add other kinds of scattering. Depending on particle size, amount and altitude, aerosols can intensify, mute or spread sunset colors. A dramatic red sky is therefore not a pure measurement of “clean” or “dirty” air.

Why the Sun itself can look red

The disk is seen through the same long atmospheric path. With more short-wavelength light removed from that direct line of sight, the remaining mix shifts toward longer wavelengths, making the Sun and nearby sky look warmer.