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 The colour of any object depends on its ability to reflect light,if the object does not reflect any colour , it appears black. The tiny droplets of water in rain clouds scatter the white light of the sun. Red is scattered first, and violet the last. This happens at such a high altitude that no rays of the sun reach the bottom of the cloud. Thus,  the rays of the sun, and this results in a dark effect.Please note that normal clouds have no water, the light is not scattered and thus look white.