The Colour of Clouds
When I was knee high to a grass hopper I loved to watch clouds. Maybe all children love to play the game of spotting animals in the shapes of the clouds. Bright white clouds on a clear blue sky look wonderful, and even as an adult it is easy to get lost in them.
But why are clouds white? The simple answer is that they reflect very well the light that falls on them. Clouds are illuminated by light from the sun, and light from the sun is seen as white by our eyes, a mix of all the colours of the rainbow - which produces white. Clouds are made up of many small water drops and ice crystals. Light can reflect and scatter so many ways from and in a cloud that when illuminated directly it ends up looking an fairly even and bright white.
Not all clouds are white. As mentioned above, the colour of a cloud depends on the colour of the light that illuminates it. At sunset or sunrise the colour of sunlight can be yellow to deep red due to the scattering of the blue component of sunlight as the light travels a longer path through the atmosphere.
A red cloud at sunset.
Red cloud at sunset.
There are also more subtle ways the colour of a cloud can change such as the colour of the light changing after reflecting off a cloud due to scattering on it's way to the observer, or due to scattering of light by air between the cloud and the observer. In the first case the blue light will scatter resulting in a white cloud taking on a yellow to red tint. In the second case a white cloud will take on a blue tint due to the blue light scattered by the air between the cloud and the observer. This is the affect that makes mountains in the distance look blue.
Clouds can also look dark or gray. This can be caused a number of different ways. But it is also due to perception by our eyes. A light gray cloud on a bright white background will look much darker than the same cloud on a dark or black background, in which case it might look white and bright.
A cloud can look dark or gray because it is partially transparent and the blue sky behind it can be seen through the cloud. This will happen in light wispy clouds with little water content, and more often in ice crystal clouds. Ice crystal clouds can spread out more as the ice crystals last longer once they move outof saturated air (air with 100% relative humidity) due to it taking longer for ice crystals to sublimate (change directly from ice to water vapor), than water drops to evaporate.
The second photo is taken with a polarizing filter which darks the background sky. In both the photos the clouds are about the same colour and brightness, but with a light background they look dark, and with the darker background they look white.
Dark clouds are also caused by shadows by other clouds, or the cloud itself. A large cumulus cloud lit from behind will shade the surface seen by the observer, and most likely shade other clouds. And again, how dark or gray the cloud appears due to the shading will strongly depend on the brightness of the background.
Green and Blue Clouds
Finally there are combination of effects. A cloud can look blue if it is heavily shaded and is illuminated by the scattered blue light of the blue sky.
Probably the most unusual colour for a cloud is green. This is thought to be due to a cloud containing a large amount of water being back lit by reddish light, such as at sunset. Water is naturally blue because it absorbs red. The light falling on the cloud appears red because the blue component has been scattered, and then the red component is absorbed by the water in the cloud, leaving green.