Rain

[rov1200]

Why are rain clouds grey to black but the rainwater is clear and transparent?

[LazyGun]

The typical white colour associated with clouds is a result of the ice crystals and water droplets that the cloud contains scattering light of all wavelengths, so the cloud appears white.
As the cloud takes on more water, or temperature changes etc, The cloud thickens as more water accumulates - This means less and less sunlight will penetrate, so giving the cloud a darker apparent colour. Additionally, cloud formations, the position of the sun and its angle will mean that the cloud base may very well be effectively in shadow.
All of these factors mean that rainclouds will look grey / black.

[seadogg]

All good stuff. As raindrops growing in the cloud each have a nucleus of dust/sulphur particles they also might appear darker in mass.

High in the mountains, the bit of cloud in front of you appears light but beyond it the cloud or mist is increasingly dark with distance.

[mibn2cweus]

mie scattering

[rov1200]

There are some conflicting results but cloud cover would prevent 15% of the suns rays reaching the earth and would stop global warming. All we have to do is produce more clouds.