When the sun is overhead (as at noon), then the light rays coming from the sun has got to travel a comparatively shorter distance through the atmosphere to reach the ground. During this shorter journey of light rays, scattering of light rays takes place but only a small amount of the blue color of the white light is scattered (most of the blue light remains in it). So the light rays coming from the overhead sun has most its component colors and with the right proportion, therefore, the sun within the sky overhead (as at noon) appears white to us.