A cloud is a visible mass of water (tiny water droplets and/or ice crystals) suspended in the atmosphere at a considerable altitude. Clouds are usually white in appearance whereas others can appear grayish and darker. From a previous post “Why are Clouds White?” it was explained that clouds are white because they scatter and reflect all the visible colors of light, which together add up to give white light. That means clouds are white because they reflect the white light from the sun.
Now, if a cloud is very thick (filled with a lot of water particles), sunlight will have to travel through a lot of water droplets before reaching the observer eye whereby a significant amount of the light is absorbed by the cloud. This will therefore cause clouds to appear very dark because of the lack of sunlight passing through it. Thus, clouds can appear dark in appearance taking on the color grey or black which will depend on the amount of sunlight passing through it, in relation to the thickness of the cloud.