Black Clouds

Ever wondered why some clouds are dark and some are white? Well I did not, but I always assumed that dark clouds bring rain and white ones just float. Anyway, today I saw an interesting thing about this on the return flight to Tokyo. As we were flying I saw a thick cloud sheet below me and patches of clouds above. The areas where clouds above were casting shadows on the clouds below were darker than the surrounding clouds. I think a darker cloud would mean that there are more layers of clouds on top of the lowest layer of cloud.