During sunrise, the light rays coming from the Sun have to travel a greater distance in the earth’s atmosphere before reaching our eyes. In this journey, the shorter wavelengths of lights are scattered out and only longer wavelengths are able to reach our eyes. Since blue colour has a shorter wavelength and red colour has a longer wavelength, the red colour is able to reach our eyes after the atmospheric scattering of light. Therefore, the Sun appears reddish early in the morning.
At sunrise and sunset, the sun is closer to the horizon. The sunlight passes through a denser layer of air and covers a larger distance before reaching our eyes. Most of the blue light gets scattered. And red colour reaches us which make the sun appears red at sunrise.
At noon, the sun is overhead. The sunlight passes through layers of air and covers a shorter distance before reaching our eyes. So, almost all colours of light are scattered equally. Hence the sun appears white.