Why Clouds Appear Grey or White

It is well know that clouds that appear during sunshine are white. But why are those that appear before rainfall grey or dark?

Clouds are formed by a group of suspended tiny water droplets or ice crystals resulting from evaporation and condensation. The colour of the clouds depends on the amount of water droplets or ice particles present in the cloud.
If a cloud contains lesser amount of water, sunlight can easily penetrate through it and make it appear bright white (reflection). However, more amount of water droplets or ice crystals in the cloud would make it thicker, denser and less penetrable by light (absorption), hence the grey appearance.