When there is sunrise or sunset the sun is near the horizon, so the light rays have to cover large distance and hence the blue colour scatters away and far from earth but the red colour scatters near the earth and lesser than blue colour because red has maximum wavelength. At noon the sun is at least possible distance from earth, so the blue colour scatters very less due to that distance and hence we see sun as white in colour, and as blue has minimum wavelength, so the rest of the colours would also not get scattered.
