Katrianna wrote about why the sky is blue in a previous article, which explained how light is made up of many different colors. White light is a combination of all of the colors. Clouds are white because the water droplets or ice crystals (at a certain altitude, the water freezes to become ice) reflect all of the colors of light in a process called Mie scattering. (All of the colors are reflected in the same way, so they combine to become white light.)
Clouds are dark when they are so thick that the sunlight is blocked by the moisture. When you look down on dark clouds through an airplane window, the clouds will always look bright white. This is because the water or ice on the surface of the cloud is still reflecting the light. Thus, every cloud will have a silver lining -- if you view it from an airplane!