Asked by Jono80
The main reason why rain clouds are dark is due to their particulate density. As clouds are formed from minuscule water droplets and/or ice crystals, the denser they are packed, the more light radiating from the Sun is scattered and dispersed by them, lending a darker appearance to their lower sections.
This is best visualised by a trip on an aeroplane, with clouds on an overcast day seeming grey/black in colour when viewed on the runway, yet a bright white when viewed from above during ﬂight. Essentially, the deeper and denser a cloud formation is, the less light will pass through it.