All clouds are white when viewed from space. However, the color of a cloud can be changed when light is refracted through the particles that make up the cloud, or when viewing clouds at sunrise or sunset.
Dust and other pollutants can also cause light to refract and change the color of a cloud.
At the surface, we often see clouds of various colors. The darkness of a clould depends on its thickness (if a cloud blocks more sunlight, it will appear darker) and also the type of precipitation it holds. Clouds that contain rain are often dark bluish-gray, whereas clould that contain hail tend to have a greenish tint.
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