Clouds form when warm, moist air rises into the upper atmosphere, where the cooler temperatures cause the water to condense. Depending on the altitude, clouds may be made up of water droplets or ice crystals, and these often form around floating motes of dust or other particles. When too much moisture condenses, the droplets or crystals become too heavy to stay aloft, falling as snow or rain.
When a cloud first forms, it is white because the relatively sparse drops or crystals reflect the full spectrum of light. As the density of moisture increases, though, some of that light is scattered back into the cloud, resulting in the darker appearance that heralds a storm cloud. The darker the cloud, the more moisture it holds, and the more likely that moisture will begin to fall.
If temperatures near the ground are above freezing, then the cloud's moisture falls as rain. Either the droplets fall to earth as water or the ice crystals melt as they fall. Snow requires freezing temperatures all the way down. Sometimes droplets begin to fall only to be boosted back up into the atmosphere by updrafts, refreezing and collecting more moisture. This process forms hail, and if the updrafts are strong enough a single hailstone could grow to significant size before falling to earth.