Shadows exist even where direct light doesn't reach, which means including them in your artwork is essential if you have any interest in realism. Many novice watercolour painters assume that creating a shadow involves only dabbling a bit of grey behind a standing object, but real world shadows are colourful, multi-tonal and multidimensional, like the objects that block the direct light and create the shadows. Two main types of shadows exist: form shadows that create contour on a subject and cast shadows that exist wherever the subject's shape blocks light.
Sketch your subject first. Include shadows in your sketch as well as areas highlighted by direct light. Note the light's direction, the shape of the shadow and any colours in the shadow, which will help you accurately depict the shadow.
Lightly transfer your sketch onto watercolour paper. Paint your subject's basic shape with a light wash of the appropriate colour and any background objects or setting elements with a similarly light wash.
Fill in the subject's shadowy contoured areas, also known as form shadows, with darker tones of the same hues. Add more of your subject's base colour to your palette to create a stronger, darker version of the same hue. Paint wet in wet, which means paint these shadows before your subject's paint dries.
Paint soft cast shadows while the subject's base paint also is still wet and allow the shadow and the subject to blend slightly. Load your paintbrush with plenty of paint and try to take a single pass at the shadow for the best results.
Paint a hard edge for cast shadows after the paint on the subject has dried. You'll want to darken your colour by 40 per cent darker at least to create a cast shadow. Notice whether the shadow is "cool," meaning it has bluish tones, or warm, meaning it has reddish tones, and add a bit of blue or red as necessary. As the shadow lengthens, soften the shadow's edge and increase the translucency of the colour by adding more water to your brush. Make sure that when you are painting your cast shadows that you do not obscure the colours beneath the shadow, which should still be visible beneath the shadow.
Translucent watercolour shades that lend themselves nicely to shadow work include ultramarine, alizarin crimson and raw sienna.