There is a lot of speculation on the web on why a cloud appears black at times.
This article will hopefully shed some light on cloud colors and the cause and effect of light refraction and how refraction affects the way we see things.
Some say that the cloud is full of water, others say it has to do with where you are in relation to the cloud. For example, are you looking at the cloud from directly beneath it or from the side of it?
In each of these cases the spectator would be partially correct.
The density of the cloud affects the way the light from the sun travels which in turn affects the way a person sees the cloud.
A person directly under the cloud may see black or grey but another person 50 miles to either side of the cloud may see light grey or even white. This is called light refraction.
A white cloud is generally not as thick or dense as a grey or black cloud, thus causing less refraction of light and appearing white. On the other hand a darker cloud has more water particles or molecules in it causing more refraction of light giving the cloud a darker appearance.
The variances in cloud color are connected to the amount of water molecules in a cloud which in turn contributes to the density of the cloud. If there is little to no water molecules in the cloud the cloud density is very low which in turn has little effect on the way the light travels through the cloud. However, if the cloud is full of water molecules this causes the cloud to become more dense. This in turn affects the speed and direction that the light is traveling. When the light changes speed and direction this causes a shadow to appear in the bottom of the cloud thus making the cloud appear a dark grey or black color.
The more dense a cloud is the less that light can penetrate the cloud cover to the surface of the planet. This is what makes the cloud appear black. However that same cloud viewed from above would be white due to the fact that you are seeing the cloud before any light is refracted.
In summary, the color of a cloud is strictly determined by the amount of refraction that takes place once light enters the cloud until the same light leaves the cloud and is viewed by you.