What makes the clouds white and then grey when its about to rain?

Answer 1

All clouds are white. The shades of grey are due to shadows. Usually the lower the clouds, rain clouds, the darker (more shadows) they are.

Answer 2

If the sunlight can pass through the clouds, then it will be white.On the other hand,if the cloud is so thick that it does not allow the sunlight to pass through,then it will look grey.
Usually,if cloud covers the whole sky,then the depth of the cloud determines the colour of the cloud; the deeper it is, the darker it becomes.
This is how grey and dark colours are seen.
On the other hand,if the sun is shining and the cloud is not covering the sky as well as the sun, then the cloud will be white in general.
Cumulonimbus are the main rain-producing clouds which are dark or grey .This is because light cannot penetate through them as they are deep and densely packed with water droplets,ice and snow particles.
Just before rain, saturation takes place and the cloud is packed with more water droplets thereby not allowing the light to pass through resulting in the grey or dark colour.

Answer 3

The amount of Rain and sunlight getting to the bottom of the cloud is what makes it's color. If it has alot of rain, less sunlight will get to the bottom of the cloud making it look grey, like a shadow.