Nothing to it.
Iu sed a flow of 5% and opacity 100%. Then I used the same brush size (about 1/10th of the circle) but different colors (it's easy to switch if you you make a few of the color your going to use. Then just use the eyedropper tool (I) to make a quick selection).
THen I just started of making a gray base and from there added the darkest color and worked my way up to the ligther.
You can change the brush mode between normal to overlay or multiply. Overlay for highlights and multiply for shadows.
Finally I duplicated the image and changed the blending mode to dissolve and set the opacity to 90%.
Oh, and call me Pete
