Question

Why is the red colour of sun at sunrise and sunset

Answer 1

Within the visible range of light, red light waves are scattered the least by atmospheric gas molecules. So at sunrise and sunset, when the sunlight travels a long path through the atmosphere to reach our eyes, the blue light has been mostly removed, leaving mostly red and yellow light remaining.

Answer 2

Sun appears red at sunrise and sunset : At sinrise and sunset, sun is closer to the horizon. The sunlight passes through denser layer of air and covers larger distance before reaching our eyes. Most of the blue light gets scattered. The light that reaches our red. That is why the sun appears red at sunrise. 

At noon, the sun is overhead. The sunlight passes through rares layers of air and covers shorter distance before reaching our eye. So, almost all colours of light are scattered equally. Hence the sun appears white