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This is because light rays cannot pass into denser medium like cloud that is filled with enormous amount of water vapor.
So as light do not pass it looks dark in color.

In case of normal cloud the density of the cloud is less as it do not contain enough water vapor and so light rays do not pass through the cloud and appears to be light or white in color.
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The light rays do not pass through the clouds. so the clouds are in black in color
where as in normal clouds  can pass because the clouds do not  have water vapoure  and then the light rays can pass trough it