Shadows do not need to be a dull plain gray. Add a little bit of color to your shadows and take your watercolors a step further. It's very easy to think of shadows as a simple flat gray. Make the shadows come alive with color.
Add a bit of local color, color from the ground, color from the sky, reflected color from the object that is casting the shadow. Add a bit of warm glow from the sun. Your painting will be livelier and more interesting with colorful shadows.