Colors of clouds
The question of cloud color is a fascinating one, and tells much about what is going on inside a cloud.
Clouds form when water vapor rises, cools, and condenses out of the air as microdroplets. These tiny particles of water are relatively dense, and sunlight cannot penetrate far into the cloud before it is reflected out, giving a cloud its characteristic white color. As a cloud matures, the droplets may combine to produce larger droplets, which may themselves combine to form droplets large enough to fall as rain. In this process of accumulation, the space between droplets becomes larger and larger, permitting light to penetrate much farther into the cloud. If the cloud is sufficiently large, and the droplets within are spaced far enough apart, it may be that very little light which enters the cloud is able to be reflected back out before it is absorbed. (Think of how much farther one can see in a heavy rain as opposed to how far one can see in a heavy fog.) This process of reflection/absorption is what leads to the range of cloud color from white through grey through black. For the same reason, the undersides of large clouds and heavy overcasts appear various degrees of grey; little light is being reflected or transmitted back to the observer.
Other colors occur naturally in clouds. Bluish-grey is the result of light scattering within the cloud. In the visible spectrum, blue and green are at the short end of light's visible wavelengths, while red and yellow are at the long end. The short rays are more easily scattered by water droplets, and the long rays are more likely to be absorbed. The bluish color is evidence that such scattering is being produced by rain-sized droplets in the cloud.
A more ominous color is the one seen frequently by severe weather observers. A greenish tinge to a cloud is produced when sunlight is scattered by ice. A cumulonimbus cloud which shows green is a pretty sure sign that someone is about to experience heavy rain, hail, strong winds, and possibly tornados.
Yellowish clouds are rare, but may occur in the late spring through early fall months during forest fire season. The yellow color is due to the presence of smoke.
Red, orange, and pink clouds occur almost entirely at sunrise/sunset, and are the result of the scattering of sunlight by the atmosphere itself. The clouds themselves are not that color, they are merely reflecting the long (and unscattered) rays of sunlight which are predominant at those hours. The effect is much the same as if one were to shine a red spotlight on a white sheet. In combination with large, mature thunderheads, this can produce blood-red clouds. The evening before the Edmonton, Alberta tornado in 1987, Edmontonians observed such clouds - deep black on their dark side, and intense red on their sunward side. In this case, the adage "red at night, sailor's delight" was clearly incorrect.