When the sun is about to set, very low down the sky, the light that is coming from the sky must travel a further distance throughout the whole atmosphere before it gets to you and reach your eyes. Along the way, more of the light is scattered and dispersed in all directions than ever before. Thus, when less of the light reaches you, the sun shines less brightly compared to other time in the day. That’s why you can see the sky changing from bright blue to dark red, as more of the blue and green wavelengths (colors) are scattered everywhere, so only red wavelengths, the longer one are able to reach in a straight line towards you in a direct beam, passing through the air molecules in the atmosphere without getting reflected in any way.