Clouds are white when liquid and ice droplets scatter the sun's rays in all different directions equally for all of the colors that make up white sunlight. The only catch is that scattered light has to reach your eyes with the same brightness as the incoming sunlight. If your eyes are below a cloud that's thick enough to absorb some of that scattered light, then the brightness goes down and you get gray clouds. Note that gray still means that all of the colors of sunlight are roughly equally absorbed.
Sometimes sunlight doesn't scatter light equally in all directions. So a thin stratus or altostratus cloud with similar-sized droplets scatters light mostly in a forward direction in a mechanism called Mie scattering. Sometimes you're in an airplane looking at these clouds, and they look grey despite them being small and thin. But somebody on the ground underneath the same cloud may see the same cloud appearing white..
Clearly cloud density and thickness affect how much sunlight is absorbed. The thicker and denser the cloud, the darker its base becomes, like in a cumulonimbus cloud (thunderstorm). Clearly someone looking at a thunderstorm cumulonimbus from the side illuminated by the sun will see a very bright cloud, because all those liquid and ice crystals are so effective at reflecting light backward and to the side.
So I mentioned scattering, reflection, and absorption. All these processes affect how clouds appear. The Mie scattering doesn't change the color of the light, but the direction has a preference for certain clouds. Reflection of sunlight is light bounced back from larger surfaces, perhaps ice crystals and raindrops. Absorption is when incident light is absorbed by an object and converted to heat, no scattering or reflection.