First the sun heats a body of water, when the water is hot enough it turns into water vapor. The vapor rises into the air and after reaching a certain point the vapor mixes with other molcules and forms a cloud. Hence after the cloud is made the molecules form into droplets, and when they are heavy enough, they fall as precipitation.
Clouds are made up of tiny water droplets or ice crystals, usually a mixture of both.
Explanation:
The water and ice scatter all light, making clouds appear white. If the clouds get thick enough or high enough all the light above does not make it through, hence the gray or dark look.
