Why are rain clouds gray?

Rain clouds are generally dark gray because light cannot penetrate them due to the deep and densely packed water droplets and ice inside the cloud. Generally, the color of a cloud depends on the cloud’s relationship to the sunlight: Clouds appear gray when they block sunlight.

The thicker the cloud, the more light it blocks. When a cloud is about 3,000 feet (914 meters) thick, hardly any sunlight will make its way through the cloud.