Why are clouds dark before it rains?

Unlike atmospheric particles that scatter more blue light than other colors (making the sky blue), the tiny cloud particles equally scatter all colors of light, which together make up white light.

However, rain clouds are gray instead of white because of their thickness, or height.

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 scatters, 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 gray as you look on from the ground below.

Source: https://www.livescience.com/