Clouds are formed when water vapor stemming from the surface of oceans, rivers and lakes evaporates, rises and reaches the dew point in the upper atmosphere. the condensation of water vapor creates the clouds. Sometimes, a cloud may appear dark or dark gray in its underside. This is commonly caused by the scattering of sunlight in the upper portion of the cloud, where visible light is scattered in all directions; letting little light inside clouds whose thickness is 1000 meters (3,280 ft.), making some cloud´s base appear dark.
Clouds are formed when the Sun´s energy evaporates water in the oceans and land. The water vapor rises and cools, reaching its dew point. Once the air becomes saturated with water vapor, it condenses, forming clouds. Most clouds reflect, scatter or absorb sunlight and usually appear white because all wavelengths of the visible spectrum; red, orange, yellow, green, blue, indigo and violet, interact with the water droplets within the cloud.
Most clouds appear white due to the scattering of visible light by tiny water droplets and ice crystals within the cloud. The light scattered by an immense number of water droplets turns a cloud white. Cloud droplets are large enough to scatter all wavelengths of visible light. Since the optical illusion created by the combination of all the colors of the visible spectrum is white color, we see a white bright cloud. As a cloud grows larger and denser, more sunlight is reflected and less light is able to penetrate the cloud, preventing sunlight form reaching the underside of the cloud, thus, making the base of the cloud appear dark.
Larger droplets at the base of the cloud may be able to absorb the little amounts of light that were able to reach this portion of the cloud, making the bottom part of the cloud appear even darker. A person standing at the opposite side of a cloud that is facing the Sun will usually see a dark cloud, while the sunlit portion of the cloud will be bright white. If a person is looking at a dense cloud from below, that person will see a dark gray cloud or a darker cloud if the cloud is too dense.
In very dense and tall clouds, such a cumulonimbus clouds, sunlight has to travel thousands of feet through the cloud before it reaches your eyes. As sunlight travels through a cumulonimbus cloud, light is scattered, reflected or absorbed, leaving less amounts of sunlight reaching your eyes. If you are observing at a cumulonimbus cloud from several km away, you will see that sunlight is reflected off the outer edges of the cloud, whereas the bulge of the cloud will acquire a gray to dark appearance, depending on how dense the cloud is.
Dark clouds usually bring rain due to the fact that when clouds get too dark, they have become big enough to hold great amounts of water droplets, which eventually fall down to the surface of the Earth as rain. Sunlight that is reflected off the clouds causes them to appear white. Denser, larger and taller clouds reflect more sunlight; however, the part of a cloud that is facing and observer receives less sunlight, making that portion of a cloud appear dark. According to edu.larc.nasa.gov, all the colors of the spectrum; red, orange, yellow, green, blue, indigo and violet, when combined create the white color. When these colors are absorbed, the color is black.