Which type of cloud is associated with rain and often appears dark in color?

Nimbus clouds are specifically associated with precipitation, including rain, and are typically characterized by their dark appearance. The term "nimbus" itself comes from the Latin word for rain, and these clouds often cover the sky with a thick layer, making them appear darker than other cloud types. Their dense and moist composition allows them to hold significant amounts of water, which ultimately leads to rain or other forms of precipitation when conditions are right.

In contrast, cumulus clouds are fluffy and white, usually indicating fair weather, while stratus clouds form in layers and can bring light drizzles but are not primarily known for heavy rain. Cirrus clouds are thin and wispy, found high in the atmosphere, and are not associated with rainfall at all. Hence, the identification of nimbus clouds as the type that indicates rain is well-founded.