Shadows are an important part of a successful painting. If you paint them at the end, they will feel like an afterthought and may muddy your colors. These last minute shadows may feel blotchy and inconsistent. Most importantly, it is difficult to add highlights at the end of a watercolor painting. Even if you can lift out color, you can never get as bright as the untouched paper. Try painting the shadows first. Once they are dry, you can over-paint the local color and your painting is complete!
Here I begin with a strong line drawing of a Bohemian Waxwing in a thicket of winter branches. Note that the line-work varies across the drawing. I use heavy lines for the outline of the bird and the branches that are in the foreground. By using lighter lines and less detail in the background, the elements behind the bird recede. Avoid using the same pencil pressure across the whole drawing.
Once the bird is sketched in, observe where the light is coming from or decide where the light will be coming from if you are drawing in the studio. Here, it comes from the upper left. Shadows will be soft on a cloudy day and hard edged in direct sunlight. Paint in your shadows using a dull purple gray. Here I have mixed Daniel Smith Shadow Violet (similar to DS Moonglow) and a little bit of Winsor-Newton Neutral Tint. Visualize the roundness of your subject as you paint. Make decisive choices keeping clean highlights. Note that the darkest part of the shadow is next to the highlight on the chest. Also there is a thin rim of light along the right side of the bird. Indicate texture at the edge of the shadow area. Do not always blend shadows to a soft feathered edge. Shadows will often end in a clean line where the planes of the subject turn.
Be careful with cast shadows. They can work for you, suggesting branches outside of the picture frame and can help describe the form of your subject. Note how the cast shadows curve across the branch and the breast of the bird showing roundness. However, cast shadows can also make your subject look like it has stripes in places that they should not be.
Now paint your colors directly on top of the shadow. Be careful to leave part of the highlights unpainted. The subject and the background will both be illuminated by the same light source. Incorporate some of the background color into the foreground subject. Here, a warm golden brown infuses the whole painting.