This is a great question (and one that I have wondered about in the past as well)! As you may have learned in your class, the reason rain is clear and snow is white has to do with the way light passes through rain drops and snow flakes (light passes more easily through rain than snow, which is why rain drops are clear while snow flakes are not).

Now, on to clouds: The part of clouds that you can see is made up of tiny water droplets (the rest is air and water vapor), and each individual water droplet "blocks" a little bit of light. As clouds get thicker and thicker, more and more light is blocked. This is why the small, thin clouds you see very high in the sky many days looks white (much like snowflakes!). However, snow clouds and thunderstorm clouds are very thick, so much more light is blocked. This means that the black color you see on the bottom of clouds is actually very similar to a shadow (like the one you cast when you block light).

Interestingly, if you could fly in an airplane over the top of thunderstorm clouds, you would notice that all of the clouds look bright white on top (since the "shadow" is only seen from below). I hope that this answer helps, and please send more questions in the future!