This is a very cool light painting. To achieve this in watercolor is fairly easy. Lay in your local colorof the tree and the, while it is still wet, lay in your core shadow. To produce light,carefully add water to the side that the light is coming from and the white of the paper and the transparency of the color will do most of the work. Then adjust to your hearts content.
This is a slightly warmer painting. I added a red-orange mix to the lights using the same technique. Very small adjustments can create enormous shifts in mood.
Think of it this way...There is an ocean of possibilities in a single drop of water,