Grey as the belly of a thunderhead, before it unleashes.
Storm Cloud is a neutral grey.
It can be described as neutral, medium and colorless.
Set in black, because black reads best on it.
Cumulonimbus is a dense, towering, vertical cloud types, typically forming from water vapor condensing in the lower troposphere that builds upward carried by powerful buoyant air currents. Above the lower portions of the cumulonimbus the water vapor becomes ice crystals, such as snow and graupel, the interaction of which can lead to hail and to lightning formation, respectively.
Read on WikipediaA storm is any disturbed state of the natural environment or the atmosphere of an astronomical body. It may be marked by significant disruptions to normal conditions such as strong wind, tornadoes, hail, thunder and lightning, heavy precipitation, heavy freezing rain, strong winds, wind transporting some substance through the atmosphere such as in a dust storm, among other forms of severe weather.
Read on WikipediaIn meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, ice crystals, or other particles, suspended in the atmosphere of a planetary body or similar space. Water, primarily, comprises the droplets and crystals. On Earth, clouds are formed as a result of saturation of the air when it is cooled to its dew point, or when it gains sufficient moisture, usually in the form of water vapor, from an adjacent source to raise the dew point to the ambient temperature.
Read on WikipediaNoun