I think you are missing the meaning of the terms that are being used in his videos. When he is referring to mixing paint, it is just that, mixing paint. There isn't a real logic to it other than adding different color paints until you get the color that you are looking for. Like adding white to black gets you a tone of gray but if you want it to be lighter you add more white and if you want it to be darker you add more black to it.
Drybrushing is painting technique that just highlights raised areas of the figure's sculpt. It is used to give the illusion of depth and shadows to figures, to make them pop more. You can lightly drybrush to get a subtle effect or drybrush several layers to get a look that really pops. All depends on your preference.
A nice tutorial on it is here:http://www.figurerealm.com/viewcustomtutorial.php?id=14