I want you to look back to some of the pictures you colored as a kid, or any of the kids you know now. If the pictures are anything like mine, the clouds will be blue. My art teacher in about 4th or 5th grade saw me doing this one day and said clouds are white, the sky is blue. Obviously I knew this, but I replied with a surprised "Oh!" as if I didn't even realize I was doing it.
I've been trying to think about why this happens. Are we trying to save our crayons? Are the rounded shapes more fun to color? Do we not want the large sky background to distract from the rest of the image? Is it just being lazy? Is it the easy way out of coloring in the whole big blue sky? (You'll probably have to ask a child and see what their answer is, when you do I'd be interested to hear the answer).
It seems that artists today still do something the quick or easy way without even realizing they're doing it. I probably have, but I don't want to. Art is most interesting to me when it challenges the viewer, and if you're the artist it's more fun to do something challenging. Where's the fun in doing the same old thing?! Go explore the big blue sky!