"Edge tone"

I have been studying this piece of work to understand how the artist made the clouds go from a dark tone to a light tone in a very short space.

The artist blended a slight mid-tone with the darker tone and turned straight to the light tone to indicate some form of "edge tone".

"Edge tone" is what I call the change in tone to suit the behind of the object. If there is a light source behind the object, then there is a bright edge tone. If it's a shadow, then there's a dark edge tone. This technique is sometimes used in movie posters where the character is shown as a silhouette.