When the sun sets, the distance between the viewer and the sun is at the most farthest. Colors have different wavelengths. Blue being the shortest wavelength, is "attenuated" leaving mostly the longer wavelenghts orange and red to reach us. The clouds in the sky and other atmospheric effects (pollution included) contributes to this phenomena.

Just a random thought, that additional shade of red and orange, as it hits the face of a love one, really does wonders. It's enough to make somebody look lovelier;)