why do rain-bearing clouds look black?

The colour of any object depends on its ability to reflect selected wavelengths of light. Consequently, 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, we do not see the rays of the sun, and this results in a dark effect. As normal clouds have no water, the light is not scattered and thus look white.