Watercolor Edge is a Layer Effect that can be added to a layer in order to give an outline to your strokes.
① Choose your colors. Your foreground color (main color) will be the color in the middle of the stroke. The background color will be the outline color.
② Tapping the i icon on the right of a layer will bring up the Layer Property menu. Selecting Watercolor edge in the effect field will set the effect. The width and strength of the outline can also be edited here.