The Sun becomes dimmer and redder as it sets and the mountain reflects this red light. Air, smoke, and dust cause this amazing transformation. A sunray beaming through thick atmosphere loses violet, blue, and green light on the way.
Red light shines through relatively unhindered.
That's why the white Sun turns red at sunset. An incident sunbeam coming into our atmosphere may smash into an air molecule or particle of dust or smoke. If it does, its blue light is three times more likely than red to scatter. The sunbeam re-radiates blue light in all directions. Less of the original light gets through because of this scattering.