Why are rain clouds dark?
Rain is water. Water is light in color. Rain clouds are full of water. Therefore, rain clouds should be light.
>>>>>>>>Therefore, rain clouds should be light.>>>>>>>>>>>
But they do not know it.
Seriously, I think the color (degree of “greyness”) depends on droplet size: the bigger, the darker. Like the deep water is darker than shallow one.
The darker the cloud, the more water is in the cloud and the less light can get through the cloud.
I think. 
First of all, water is NOTHING in color (as long as you’re not too close to Los Angeles). It is clear. But when you get a lot of it together, it looks blue, doesn’t it?
Same principal with clouds. You get a lot of water vapour in the clouds and the density of the clouds compared to non-rain clouds and it’s like comparing the glass of water to the ocean.
Yer pal,
Satan
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For the same reason it’s dark at the bottom of the sea (and why deep water is dark blue at the surface):
Water is an attenuator; it absorbs light.