The atmosphere contains free molecules and atoms with different sizes. These molecules and atoms scatter light of different wavelengths which are comparable to their size. Molecules having a size that is comparable to the wavelength of red light are less is the atomosphere. Hence scattering of red light is less when compared to the other colours of light. The light from the sun needs to travel more distance in atmosphere during sunrise and sunset to reach your eye. In morning and evening times, during sunrise and sunset, except red light all colours scatter more and vanish before they reach you. Since scattering of red light is
very small, it reaches you. As a result sun appears red in colours during sunrise and sunset.