Painting and drawing with the Art Media tools : Using the Watercolor Brush tool
 
Using the Watercolor Brush tool
You can use the Watercolor Brush tool to simulate the transparent strokes of watercolor painting. Watercolor brushstrokes reveal the canvas texture and are wet in terms of how they interact with other strokes when you smear or mix them together. For more information see To dry or wet an Art Media layer.
To reload the Watercolor Brush tool with color, you can
reload the brush with a solid color
reload the brush by sampling from the Mixer page
Watercolor Brush tool options
Edit workspace 
Watercolor Brush tool options include
Shape — defines the shape of the brush tip
Size — defines the size of the tool head in pixels
Thickness — defines the aspect ratio of the tool head. This setting is active only when the Fixed angle Head Tracking option is chosen.
Rotation — defines the angle of head rotation in degrees
Head Tracking — determines whether the tool head bends around the path of the user’s stroke, or remains at a fixed angle
Viscosity — defines the rate at which the material on the brush is applied, and thus the length of the stroke before your paint runs out
Firmness — defines the amount of splay in the brush, how the rendered line gets wider with more pressure, and how well it penetrates the surface on to which it is painting
Bristle size — affects the underlying noise function for texture and application of paint
Auto Clean check box — cleans the brush and reloads it with fresh paint at the start of a new stroke
Note: When unmarked, the brush head is not cleaned at the start of the stroke. Instead, the dirty head has a small amount of the current color added to whatever dirty head state exists from the previous stroke.
Clean button — cleans the head and start the next stroke with fresh paint or pigment
Trace check box — allows the current Art Media tool to select the pigment or paint color by sampling the data below the center of the brush regardless of the layer type.