Small particles of dust and pollution in the air can contribute to (and sometimes even enhance) these colors, but the primary cause of a blue sky and orange/red sunsets or sunrises is scattering by the gas molecules that make up our atmosphere.

Long story short,, the answer to ‘why is the sunset red?’ is: At sunset, light must travel farther through the atmosphere before it gets to you, so more of it is reflected and scattered and the sun appears dimmer.

The red colour we see during a sunset is namely due to the scattering of light from the sun off of particles in the atmosphere.

Sunsets are reddish because the sun is not directly overhead and it's rays must cross through much more atmosphere than the midday sun. After having crossed so much air, most of the blue light is scattered out, as well as most of the green.