Why is the sky blue, and clouds white?
It's not that everything is lit up by blue light. No, sunlight is actually white. But the sky is blue. Where does this color come from? And think about this: the sky is only blue if there are no clouds, fog, dust, and haze; that is, if there are no particles in the air.
The sun's light contains all existing colors, which together appear white (see the article Where do Rainbows come from). All the colors are the sunlight's visible spectrum. It ranges from red to violet. Somehow, the blue of the sunlight gets separated in air, which is what we observe as a blue sky.
How do the colors of the sun get separated in air?
When sunlight goes through the earth's atmosphere, it gets scattered by particles and even by tiny air molecules. Scattering means that the light hits a particle or molecule and gets reflected in a random direction. In clean air, there are few particles, so the light gets scattered mainly by air molecules such as nitrogen and oxygen. Air molecules by themselves scatter blue light stronger than red light. If we look at the sky, the light we see is the sunlight that was scattered by a molecule before it reaches our eye (see figure).
Because blue light gets scattered by molecules more than any other color of light, the light which reaches our eye contains more blue. And thus, if we look at the sky, it appears blue. This is why a blue sky means that the air is quite clear. If the sky is not blue during the day, it means the air is dusty or foggy.
The situation is different when we look at the sky during sunset. The sunlight has to travel further through the atmosphere to get to us. Then, the blue light gets scattered away and we see more unscattered light, which is more red. So the evening sky is red instead of blue.
So why are clouds white or gray?
In clouds, we have water drops, which are much bigger than the air molecules. The sunlight gets scattered here too, but bigger particles scatter light almost the same way for each color of the spectrum. Also, because there are so many drops in a cloud, the light scatters a few times when it hits clouds. So in the light which reaches our eye, there is not more light of a specific color. That's why we receive the complete color spectrum when we look at clouds. And the complete color spectrum together is white.