The other colors travel to the ground with little interference. The blue light is scattered from molecule to molecule in the sky, until the light seems to be coming from every direction. The clouds scatter and reflect all the visible colors of light that strike them. Hence, we have white clouds. But if the cloud is thick enough, light does not penetrate completely through the cloud, resulting in dark, heavy-looking cloud bottoms.
Clouds are nothing more than water vapor that condenses and accretes into a visible form.
The usual mechanism is for moisture-laden air near the Earth's surface to be raised higher into the atmosphere either by an encroaching air mass or the heat of the sun. As the air is lifted, the pressure drops and the air is subsequently cooled. The combination of the two causes water vapor to condense.