Sunsets look red because the sunlight travels through more atmosphere, scattering away blue light and leaving the longer red wavelengths.

When the sun is near the horizon at sunrise or sunset, its light passes through a thicker slice of atmosphere. The shorter blue and violet wavelengths get scattered out of the direct path to your eyes, so they don't reach you as much. This leaves the longer red, orange, and yellow light to come straight through. That's why sunsets glow with warm colors. Dust and pollution in the air can make these colors even more intense by scattering blue light further.