Question

why cloud which are going to rain are seem black

Answer 1

Explanation:

As the become heavy and don't allow light to pass through them

Answer 2

When clouds are thin, they let a large portion of the light through and appear white. But like any objects that transmit light, the thicker they are, the less light makes it through. As their thickness increases, the bottoms of clouds look darker but still scatter all colors. We perceive this as grey...

Hope this helped....