To develop the shadows even further, I have used a very wet puddle of VanDyke Brown and applied it to a few areas where I want darker or more striking shadows.
One theory I like to use for shadows is that if I glaze a relative opposite of the local color, the colors will visually blend (not physically blend) to create a darker value. For instance, if I want the dark green shadows to be very dark in value, just adding green will only make the paint thicker, it won’t add to the darkness of the value. By adding a relative opposite, like a red or warm brown, I can add to the value, then glaze over it with the local color (green in this case) and the green becomes what the viewer sees, but the warm tones add darkness to the value. Hope this makes sense.
Think of it this way: relative opposites on the color wheel, like red and green, when physically mixed will make a neutral, dark gray. If I glaze these colors they will still VISUALLY make a dark, more neutral color, but they will retain some of their more pure color characteristics. Try it on some scrap watercolor paper. Glaze a red, allow it to dry, then glaze a green. The green will still look green, only darker!