Why do clouds turn gray?
Clouds are made up of tiny water droplets or ice crystals, but usually a mixture of both.
The water and ice scatter all light, making clouds appear white.
If the clouds become thick enough or high enough, all the light above does not make it through the cloud. Therefore, it looks gray or dark.
Also, if there are lots of other clouds around, their shadow can add to the gray or multicolored gray appearance.