Why is the sky blue?
When sunlight passes through the atmosphere, it bangs with gas molecules. These molecules spread the light. The shorter the wavelength of light, the more it is dispersed by the atmosphere. Because it has a shorter wavelength than the other colors, blue light is scattered more, ten times more than red light, for example. That is why the sky is blue.
Why does the setting sun look blood-red orange? When the sun is on the horizon, its light takes a thirster path through the aura to reach your eyes than when the sun is straight overhead. By the time the light of the setting sun reaches your eyes, most of the blue light has been scattered out. The light you finally see is reddish orange, the color of white illumination minus blue.