Nimbus clouds are typically gray or dark gray, sometimes even appearing almost black.

Here's why:

* They are thick and dense: Nimbus clouds are made up of water droplets (and sometimes ice crystals), packed tightly together. This density blocks sunlight, giving them their dark appearance.

* They often contain precipitation: Nimbus clouds are rain clouds, and the water droplets within them are large and heavy. These droplets further contribute to the cloud's dark color.

While they are usually gray, nimbus clouds can occasionally have a lighter, more bluish hue, especially at the edges.