why are clouds white, Grey, dark grey or nearly black?
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Water is transparent, but it does *refract* light. That is, it bends light like a lens, shooting it off in other directions. Sunlight that *was* pointed straight down is now being scattered in many directions, and much of the light ends up not going down anymore. That creates a shadow under the cloud, making it appear darker.
Thinner, less dense clouds scatter the light less, so more of it gets down to you. However, it does still scatter the light, which mixes to what we perceive as white light. Additionally, although the ground is not very reflective, it does still reflect light back up into the sky. Sunlight passing around the clouds will bounce off the ground, hit the bottom of the clouds, and get refracted/reflected back down as white light.
Thicker, more dense clouds scatter more light. Every photon that does end up pointed downward has to go through more water droplets, which have more chances to scatter it in another direction. Very tall stormclouds produce ice crystals and hail, which further scatter the light back up and away. The bottom of a very tall stormcloud is going to be dark, maybe even black. A big storm also covers more of the sky, so there is a lot less sunlight spilling out from *around* the clouds, so less light reflects back up to the bottom of the cloud to illuminate them.
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