Clouds are grey because of our perspective from earth.
Since the sun is shining from above, clouds are lit from the top. That's why they always look white from an airplane no matter how thick they are. Looking up at clouds from the ground, thin clouds allow most of the light to pass through, so they look white. But as clouds get thicker, they block a lot of the light. Since light doesn't make it all the way through, the bottom of the cloud looks dark.