"Why are some clouds dark?"- Max and Logan Sentner
This depends mostly on the density, or thickness, of the cloud. Clouds are made up of tiny water droplets that reflect the sunlight. As the light passes through a cloud, it reflects evenly off of all the droplets making it appear white to our eyes.
As clouds build vertically and get thicker, such as a cumulonimbus cloud, less light can pass through the cloud. This will give it a darker appearance. This is also the reason why the bottom of clouds sometimes appear darker than the top.