In daytime, why are some clouds bright white and others dark grey?

From the ground, looking up at clouds is a pleasant pastime. Some find joy in seeing abstractions of familiar shapes, while others watch motion and the depth of shade.

Although many notice some clouds look very bright white and others are dark and foreboding, few question why. After all, each cloud is made up of the same materials: water vapor, droplets of water, and ice crystals. Why the obvious difference in shade of white? To start from the brightest, a thin, wispy cirrus cloud will always be bright white in the daytime. Thicker clouds will have brighter edges and darker central portions, due to reduction of light penetrating through the cloud. As clouds get larger and thicker, like a thunderhead, the thickest section at the center is very dark. Almost no light is getting through that part of the cloud from above.