When I first started painting, mixing colors was one the most frustrating challenges. I didn't know what I was doing so, I was wasting a lot of paint. I understood the basics of the color wheel but, I couldn't get the more elegant and sophisticated colors that I wanted. Especially when working on a landscape, a particular shade of green or brown could really change the intent of the painting.
I started looking for books and classes and over time, every little bit of advice helped but, one resource I used often and still go back to is my "Color Mixing Recipes" book. It's such a great, easy reference to use.
The book starts with some basic instructions and background about color theory but, most of the book are "recipes." Recipes are organized by color and value. Each color has a recipe underneath it.
I simply find the color I want and then follow the recipe. The numbers in the recipe represent "parts" which depends on how much paint you need. It could be a smidgen or a dollop. If the recipe says "6 white and 2 cobalt violet," then you use 6 parts of the color white and 2 parts of cobalt violet.
Even if you don't have the exact tube of color you can usually improvise. The suggested recipes are a great starting point when you're not quite sure how to start. Towards the back of the book there is a spread on portrait colors, which is especially informative. The nice thing is that he provides suggestions for a variety of skin tones.
The book is listed for a little over $5.00 on Amazon, which is a great price for such a useful reference book. There are other titles in this series but, I've only owned the small one for oil & acrylic and it is definitely worth it.