Why are clouds white
The reason why clouds are white is quite similar to the reason why the sea or the sky is blue. Sunlight is “white light” and this white light is made up of the seven principle colors of the spectrum, which are violet, Indigo, blue, green, yellow, orange and red. The “vibgyor” wavelengths together travel in sunlight and appear white. Usually the reason why an object appears to be of a specific color is because it reflects that particular wavelength of color only. Sunlight falls directly on the clouds and passes through them to reach the surface and ultimately to human eyes. As a cloud is made up of water droplets and ice crystals, none of the color wavelengths travelling within the sunlight is absorbed beyond visibility by the clouds, thus the reflected sunlight reaches our eye in the same intact, “white light” form.
Clouds that are perfectly white usually are located high up on the sky, where the cold temperature freezes almost all the water droplets into ice crystals. All clouds are not white though or at least they do not appear white. The thickness of a cloud actually is a big determinant of the shade of the cloud. We all know that storm clouds are dark in color, but the real reason behind its darkness is its thickness or density which blocks most of the sunlight from reaching our eyes thus making the cloud appear dark in color.
Grey clouds are not necessarily rain clouds and the grey color is often because larger clouds high above cast a shadow onto smaller clouds below them. Apart from this, even a big, dense cloud can cast a shadow of its own making it appear grey in color. Depending on the position of the sun in the sky, a cloud may also appear blue. Clouds may also appear red and orange during sunsets.
Although clouds appear white, it is only the reflected and unabsorbed white light from the sun that lends them their white color. Even the storm or rain clouds would appear white if you get a chance to look at them from above them.