I was just asked if I could explain how I choose colours to mix for shades and highlights. Well, a lot of it is my art training, and just understanding colour theory. The rest is experimentation.
However, if you need a place to start, then the chart here is a perfect place to begin! Feel free to save this chart if you're using P3's and want a quick guide for when you're trying to find good shades and highlights for certain colours.
This is a guide. It's not Gospel.
Some of these combinations produce....unpleasant results. But, for the most part, what is on this chart works well, and is an excellent beginning point for when you're ready to start making your own custom mixes.