>>5069
If you're trying to mix a brown, generally you do so by mixing complementary colors together depending on what hue you want. When it comes to mixing, it ultimately depends on what you're looking for specifically, so it's best to do some studies with mixing.
But if you want a more in depth look at color theory, The Munsell Color System is a good place to start. You can find it online, and a good exercise for learning to mix colors better is to try and recreate it from scratch on your own.
I would also recommend, if you can find it, Josef Albers' "Interaction of Color" if you wish to see the above principles in action. It's also a good resource for knowing how colors are effected in the presence of other colors.