Why are clouds white and why do they change colour when it's about to rain?

Clouds are white because they reflect a large portion of solar radiation, and absorb the rest. Very little of the sun's rays are transmitted although some still manage to pass through and warm the ground beneath. Why they are white is the same explanation as to why snow is white although the individual crystals are translucent.

Here, individual water droplets in the clouds allow light to penetrate as well and are certainly not white but overall, because of all the absorption and internal reflection within the droplets, the net effect produces the white color.

Denser clouds appear grayish because even less of the Sun's radiation can be transmitted through them hence they appear dark to an observer beneath. But to an observer above the clouds, they are still white.

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