Nine answers and only one is correct, sad. It was pretty much said, but I'll elaborate; when the sun's light enters our atmosphere it collides with air molecules, which because of their size, mostly adsorb short-wave length light such as red and green. What is left over is the long-wave length blue light. This is consequently why plant leaves are often green and red but never blue, they are trying to get the most common wave length to be adsorbed.
The color of water molecules is what I always thought. Because you know.. Water evaporates into clouds, wait.. Then why are clouds white? Clouds aren't the sky.. Maybe the sun reflects off of something to cause it to be blue. And now, I'm lost and don't know what else to say but think about what I just said and find an answer.