At the time of sunrise and sunset, the light from Sun has to travel the longest distance of atmosphere to reach the observer.
Since, the blue light of short wavelength is scattered more, much of it is lost, while the red light of long wavelength is scattered a little, so it is not much lost.
Thus, blue light is almost absent in sunlight reaching the observer and only the red (white - blue = red) light reaches us.
As a result, the Sun and the region near by it, is seen red.