You are correct! Most rain clouds are dark because they are filled with water droplets. Sunlight reflects off these tiny drops of water, making the clouds appear darker than they actually are. Clouds can also look dark when they are thick and close to the ground. 

However, rain clouds are gray instead of white because of their thickness or height. That is, a cloud gets thicker and denser as it gathers more water droplets and ice crystals — the thicker it gets, the less light that can pass through it. When a cloud gets thick enough, virtually all of the sunlight is blocked out and the cloud appears gray or dark.

I hope that helps! Let me know if you have any other questions.
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