It is mostly reflection that accounts for the whiteness of a cloud. Absorption explains why some clouds are darker.
Work Step by Step
The large droplets of water in a cloud don't scatter light as effectively as the small molecules of oxygen and nitrogen do. They mostly refract and reflect light and treat all colors the same, resulting in a white cloud. These large droplets absorb some of the light that falls upon them. If the cloud is big enough, there are many of these droplets, little light makes it all the way through the cloud, and the cloud appears dark. If the droplets become larger still, they turn into rain. This is discussed on page 514.