The color we see in clouds, the sky, or your clothes is determined by how light is transmitted (filtered) or reflected (bounced back like a mirror) by these objects.

Objects that are white, or any shade of grey, reflect or transmit equal portions of the color spectrum. You can see the colors of the spectrum, which range from red to yellow to green and blue, with a prism or in a rainbow. When an object absorbs some colors but reflects others, we see the color that is reflected. For instance, an orange absorbs blue light, but reflects red and yellow light so we see the color orange.

Clouds both reflect and transmit light from the sun, but they do not absorb any one of the colors we see much more than the others. So we see white when the clouds are thin and well lit, grey when they are thick, and almost black when they are very thick and block most of the sun's rays.

You are probably curious now as to why the sky is blue instead of white. That's because unlike clouds, the atmosphere scatters blue light more than the other colors of the spectrum. That scattered light from the sun is what we see when we look up at a clear blue sky.

So, here are a couple of questions for you to think about:

Why are sunsets red?

What color is the ocean when it is cloudy? Sunny? Why?

Clouds are basically white because they scatter all wavelengths of light equally (in contrasts to the clear sky, which preferentially scatters blue wavelengths and lets the redder ones pass through).But, a thick cloud will be dark white, or gray, because it blocks a lot of the light! Does that make sense? I'm not sure I put that very well. Try this: When sunlight hits the top of a cloud it is scattered in all directions. Some gets scattered farther into the cloud. This light is scattered also, and a fraction of it is scattered farther into the cloud. The deeper into the cloud you go the less light is received from scattering above. In this way, the thicker the cloud, the less light gets through it. So, if you are looking at a whispy thin cloud it looks white. A thicker cloud looks gray, and a very thick thunderhead (cumulonimbus) will be almost black at the bottom because hardly any light gets through it.

Clouds are actually white. When you look up and see grey, remember that you are looking at the undersides of the clouds. They appear grey because they block out part of the sunlight. They do this by scattering light back out into space so that it doesn't reach your eyes. With less light, the cloud looks grey.