At the time of sunrise or sunset the sun is near the horizon hence the sunlight has to travel the maximum distance to reach us. Due to this large distance, the most of the shorter wavelength blue-colour present in the while light gets scattered out and away from our line of sight, leaving behind mainly red color in direct sunlight beam that reaches our eye. Hence the sky appears red at the time of sunrise and sunset.
At noon the sun is overhead in the sky. The light coming from the sun has to travel a shorter distance through the atmosphere to reach us. Due to this short journey, only a little blue light gets scattered. Since the light coming is having almost all the components in right proportion hence the sun in the sky overhead appears white to us at noon.
