This is an easy one, perfect for a Monday while I’m still rubbing my eyes open:
I can’t believe this wasn’t nailed right away in the WikiAnswer (although, admittedly, the answer posted there does make a lot of sense).
Clouds are white because they are God’s cotton ball supply. I don’t know about you, but I’ve never seen a non-white cotton ball. He plucks a handful of cloud and rolls it up and then proceeds to do whatever it is you do with cotton balls (what do you do with cotton balls?).
They also serve nicely as the ends of q-tips and ammunition in heavenly snowball fights.