The Mixer Brush tool in Photoshop can make for realistic painting, and can result in far more natural-looking brush strokes
One of the great things about the Mixer Brush tool is that you can load not just one colour but multiple colours to paint with.
To load multiple colours across the brush tip, all you need to do is simply Opt/Alt-click when using the brush. You can do this either in an existing painting, or by simply laying two colours side by side on the canvas and Opt/Alt-clicking over the point at which these colours meet.
This can be especially useful if you need to carefully blend one colour with another.