During noon days, the sun is almost at right angle over our head. So, the white sunlight has to pass through an atmosphere of lesser distance as compared to the time when the sun is slanting in the evening or during sunrise. As white light is made up of seven different colors among which violet and blue is scattered the most and orange and red the least, when the sun is at sun-rise/set, we see it red as only the red light reaches us and blue is scattered in the way. However, in noon, the sunlight travels the least to reach us and we can obtain almost every of the visible spectrum. So, it appears white. Now, as the cloud particles are of size equal to that of wavelength of white light, they also appear white. So, as both un and clouds appear white, we cannot distinguish the sun in the noon-sky.