It turns grey because they get denser and heavier.
cirrus clouds are dark and usally bring rain
cumulonimbus clouds
Nimbus clouds or thunderclouds are usually tall, dense and very dark in color. These clouds form thunderstorms and heavy rain usually follows.
Clouds are only gray on bottom when they are ready to let go of all of the moisture and pollution in them. which means they are gray when it is going to rain.
Condensation. That is how. Water evaporates, forms into clouds, and when they get too full of water, it falls back down as rain. White clouds never rain. Gray clouds will sometimes rain. The darker the cloud is, the more likely they will rain. The grayest/ darkest ones will undoubtedly also storm.
Nimbus clouds are typically associated with rain. They are thick and dark clouds that often bring precipitation in the form of rain, snow, or hail. When you see nimbus clouds, there is a good chance that you'll experience some kind of precipitation.
Thick low clouds that are a sign of rain are called nimbostratus clouds. These clouds are typically gray and cover the sky, producing long periods of steady precipitation.
Nimbostratus clouds are thick, low-level clouds that bring steady rain or snow. These clouds often cover the sky in a blanket of gray or dark gray, and the precipitation they produce can last for a long period of time.
Not trying to be funny but a bunch of gray clouds is probably all your going to see.
Clouds turn gray when they become thick and dense, blocking sunlight from passing through. This reduces the amount of light reflected off the cloud, making it appear gray to our eyes. Gray clouds typically indicate that rain or a storm may be approaching.
Stratus clouds are typically associated with light rain and drizzle. These low-level clouds form a thick, gray layer that can produce steady, light precipitation over a large area.
Low, shapeless rain clouds are often dark gray in color, indicating that they are dense and likely to produce precipitation. These clouds, typically known as nimbostratus, lack the defined structure of other cloud types but are thick enough to block sunlight. Their dark appearance is a result of the moisture content within the clouds, which can lead to steady, continuous rain.