Question

Why do clouds appear grey-black before rains?

Answer 1

Rain clouds are grey instead of white because of their thickness. That is, a cloud gets thicker and denser as it gathers more water droplets and ice crystals — the thicker it gets, the more light it reflects off, resulting in less light penetrating all the way through it. The particles on the underside of the rain cloud don't have a lot of light to scatter to your eyes, so the base appears grey as you look on from the ground below.

This effect becomes more pronounced the larger the water droplets get — such as right before they're large enough to fall from the sky as rain or snow — because they become more efficient at absorbing light, rather than scattering it.