MEMPHIS, TN (WMC) - It is all about how thick the clouds are and how high up they are. Clouds are made up of tiny water droplets of water or ice or the combination of both.
The tiny water droplets and ice crystals in clouds scatter all colors of light, and when light contains all colors, we see it as white. When clouds are thin all the light is able to penetrate through but when clouds are high and thick some of the light is not able to go all the way through. As the clouds get thicker, the flat bottoms of the cloud can still scatter all light but because of the height and thickness, we see it as gray.
Another reason that clouds look gray can be due to other clouds. The more the clouds, the more the shadow of other clouds may make some clouds look gray.