Sun appears red at sunrise and sunset : At sunrise and sunset, sun is closer to the horizon. The sunlight passes through denser layer of air and covers larger distance before reaching our eyes. Most of the blue light gets scattered. And red color reaches us which make the sun appears red at sunrise.
At noon, the sun appears Whtie : At noon, the sun is overhead. The sunlight passes through layers of air and covers shorter distance before reaching our eye. So, light undergoes the least scattering. Hence the sun appears white.