As we know with the air around us filled with water in the form of gases or water vapor, therefore cold air rises above the higher atmospheric layers and causes water vapor to condense this process is called cloud formation.
clouds absorb all the light, so as water vapor forming rain or snow gather more and more, the resulting dark color (gray), this happens because the thickness of the water layer in the cloud. That means, the darker the clouds we see, the thicker and higher the water is.
@themotor