Painting brush with a strong sub color and butter-smooth blending.
Also makes a decent soft lineart brush.
This entire picture was made using this brush (plus a little bit of gradient map effects.)
The sub color easily overwhelms the main color, so select the sub color if you want to paint with a simple, predictable color.
The main color is useful for creating subtle variations on a sub color.
Two colors can be blended together very smoothly with a few gentle back-and-forth strokes. More pressure and indirect strokes produce a blockier transition between colors.