Clouds darken from a pleasant fluffy white just before rain begins to fall because they absorb more light.
Clouds normally appear white when the light which strikes them is scattered by the small ice or water particles from which they are composed.
However, as the size of these ice and water particles increases, as it does just before clouds begin to deposit rain, this scattering of light is increasingly replaced by absorption.
As a result, much less light reaches the observer on the ground below and the clouds look darker.
A cumulonimbus cloud emitting green is an imminent sign of heavy rain, hail, strong winds and possible tornadoes.