ulonimbus Clouds?

The apparent colour of clouds depends on where you are in relation to the Sun. Comulonimbus clouds are very thick; sometimes the base is about 2000 feet and the anvil 60 000 feet above ground level. If the Sun is high in the sky, as it often is in the tropics, then part of the Cb cloud is in shadow and appears almost black. But even in the most polluted places cloud droplets and the air between them are almost colourless.