My late wife, who was fighting ovarian cancer at the time, painted clouds on our bathroom ceiling in shapes representing pets or ideas from our family's history. We had the splattered acoustic material type surface on the ceiling, and it was painted white throughout the bathroom - just plain 'Kilz' white.
In the center of the ceiling was a brass-themed globe light, which ended up being more or less like the sun in the sky. To end up with clouds, she simply took sky blue paint and applied it over the white paint with a big sponge.
Instead of painting in the shapes of the clouds she wanted, she painted in the sky between the clouds. We're not talking DaVinci here, but it was beautiful to me.