I have often been asked "how do I mix and paint a shadow?" I just thought I'd share it here for those who might be having a problem with it. Before you just reach for the Paynes gray or black and white, maybe consider this. It's a time tested method used by artists for hundreds of years.
There are basically 2 types of shadows. Object shadows and Cast shadows. This method will help you to create a shadow for each.
Take a standard color wheel. Every art store should have one if you need one.
Now look at the color of the object in your painting that you want to create a shadow for. (Try to get it as close as you can by eye) Match it to the color closest to it on the color wheel.
Now, Look directly across the wheel(180 degrees) at the "complimentary" color to the one you are using.
Mix or find that complimentary color and add just a speck of it to your objects color on the palate. You will notice that a slightly darker value of your color appears. You can adjust the value by adding more.
This works well when you want to shade something without adding gray or graying the object. It takes a little practice but it isn't difficult. You can mix it on the palate or even scumble it in on the canvas and adjust it as you go to create an increasingly darker shadow value. Hope it has helped some.