But rain clouds tend to be gray, what's this all about? Firstly, it rains when water and ice droplets in the cloud get too big to be held up by the rising air. Clouds containing all this water can get pretty big and thick. When a cloud gets to about 3000 feet thick, it will not allow any light through and this is what happens with gray clouds. So the sun will be shining on the tops of the clouds, making them look white if you happen to be in a plane but won't reach the bottoms, so the clouds look gray to us on the ground. It also depends on where you are standing - if you're right under the cloud its going to look pretty dark but as it moves away, the side is illuminated by the sun and it will look white.
Also, storms often happen in the late afternoon when the sun
is no longer right above us. This means that clouds will have
a good chance of getting between us and the sun so we won't get
much light and the cloud will look darker. Clouds can even look
black - this happens with tornados. These clouds are so big and
dense that they block out all light from the sun.