Why are rain clouds darker?

Rain clouds are dark because the part of the cloud you see is in the shade.

Clouds are white because they contain tiny water droplets that scatter light of all colors equally in all directions. "Scatters light of all colors equally in all directions" means "white".

But if you put a layer of white stuff over another layer of white stuff, the top layer will scatter light from the Sun, reflecting a lot of it into space. That means there's less left to light up the layer underneath. Compared to the top layer, the bottom layer will look darker.

For a cloud to produce rain, it needs to be fairly tall (thick). That means the upper parts of the cloud reflect away most of the sunlight, leaving the lower parts in the shade. If you're under the cloud, the lower part is all you see -- and it looks dark.

This is due to the scattering of light.As the density increases scattering increase and light rays coming from sun dispersed among them. So the lower part of that clouds appear as darker but above part will be white, you can observe it on your flight, but you have to grab a window seat for a beautiful view