These clouds were interesting to paint.
I started with a background, which was a round gradient from orange to gray, sort of like a sunset on a cloudy evening. First I tried using just the Smudge tool to make clouds, but that was taking too long. So I just painted in some cloud shapes, then swirled them with Smudge afterward. I tried changing the brush on the Smudge tool, and got some really great effects. And just with the default Photoshop brushes, too! I had fun making them all varied and cool looking.