A cloud compsition is primarlly just billions of drops of water or ice.  When such a cloud formation has an influx of water/ice, ie it's mass increases to include more water/ice, light finds it hard to pass through the mass, thus making it darker for us to see.\n\nIn a normal cloud, light can pass through the water/ice within the cloud and we see it as a distorted white/bluish/greyish/greenish hue.  But when the cloud is too thick with water/ice, the light doesn't pass through so easily and we see a darker spectrum.
why do clouds turn dark when its about to rain?