Why are clouds black color when going to rain?

Well dear I think the number one respondents answer your question substantially enough so now you must know already why clouds turn dark when it its going to rain. And I think it is really good that clouds changes color when it is about to rain because at least God is giving us a good signal that we are outdoor we have to rush to reach home or get shelter lest we get wet when the rain fails. If the cloud remains as white as though there is no sign that it would rain surely it is not to our benefit. Don't you think so?

due to the heat generated by the sun and carry water from the sea and then after the build up in the clouds, the clouds that had been white would turn black from too much water

when will the rain, clouds will be highly concentrated water molecules. These water molecules bind to dust in the air to form a gray color. as additional information, the cloud will turn into drops of rain since this dust mixture. with the mixing of dust, water molecules in clouds become heavy and fall into the rain.