Have you noticed that clouds change colour over time? They start off as white and fluffy and then turn to heavy, dull, grey clouds. Isn't it strange?! There are a few different ideas on this and here are some of them!
Some people say that the heavier the cloud is, the darker it gets. This may be because the sun can't get through the heaviest clouds so makes them darker. After evaporation occurs, water drops form around dust and when they get heavy enough, it rains! This happens everywhere around the world but a lot more often in some places *cough*England*cough*.
Other people say that clouds look grey when we are directly underneath them. This might be because the sunlight doesn't shine directly on the bottom of clouds, but through the top of the clouds.
The sunlight has to travel a very long way to get to our eyes. As the light travels through a thick cloud, the light will get scattered and reflected off things so when the light eventually reaches us, it will look different.
If all the colours in the spectrum are there (red, orange, yellow, green, blue, indigo, violet) then the colour of the light is white. If none of the colours of the spectrum are there, the colour is black. This may also be why clouds look different colours. It just depends what colours can get through the clouds!