Cloud

The color of a cloud tells much about what is going on in the interior of the cloud. Clouds form when relatively warm air containing water vapor is lighter than its nearby air and this causes it to rise. As it rises it cools and the vapor condenses out of the air as micro-droplets. These minute particles of water are fairly densely packed, and sunlight cannot go through far into the cloud before it is reflected out, giving a cloud its attribute white color. As a cloud matures, the droplets may join to produce larger droplets, which may themselves merge to form droplets large enough to fall as rain. In this process of accretion, the space between droplets becomes larger and larger, permitting light to enter much farther into the cloud.

If the cloud is satisfactorily large, and the droplets within are spaced far enough apart, it may be that a percentage of the light which enters the cloud is not reflected back out before it is absorbed .This process of reflection/absorption is what leads to the range of cloud color from white through grey through black. For the same reason, the undersides of large clouds and heavy overcasts appear various degrees of grey; little light is being reflected or transmitted back to the observer.