|Cloud Colours & Rays imaged by Ábrahám Tamás (site) at Zsámbék, Hungary. ©Ábrahám Tamás, shown with permission.
Rays and shadows seem to radiate in all directions from the backlit cumulus. They are visible because the air is hazy and scatters light well. The rays are really almost parallel.
The cloud is dark because it is thick and light is scattered many times by droplets. The portions away from the sunlit side are shadowed. Parts of the extreme edge are thinner and forward scattering by their droplets produce a 'silver lining'.
The cloud is not a pure gray. It has subtle colours that painters look for. Skylight, reddened sunlight, scattered light from other clouds and light from the ground and water all contribute blues, purples and reds. Parts look greenish but that is mostly an illusion created by other nearby colours.
The squares show colours directly sampled from the cloud image. At left the background is neutral gray. That at right is one of the sky colours.