To me, this piece tells me color mixing is not thaaat important. When you can sample from a mix of colors on a palette.
@Sydney you can find some clone brushes from here:
Pick up an area color for painting like palette Knife - #13 by tachiko
The ones in green border are clone brushes as indicated by picture in the Tumblr link
Also on the brush preset editor there is a collapsed part to the left that can be revealed by pressing arrow which reveals all the Brush Engines to choose from