You only need three colors of paint to make a painting.   If you have red, yellow and blue, you can create almost anything you need. All the other colors make painting easier, but they aren’t necessary.
Red, yellow and blue are called the “primary colors†because you can’t get them by mixing any other colors, but they can mix every other color. So if you’re a beginning artist, don’t go crazy buying colors at first. You need enough colors to be helpful and practical, but not so many that you have a tube of paint for each color! Mixing colors from other colors is best because it will give your painting more variation and harmony.
To mix color, it is not like going into a paint store where you mix by a formula. As you become experienced at painting, you will learn to mix colors by analyzing the color you want. It’s a bit like singing; if a good singer hears a note on a piano, he or she can create the same note without thinking much about it. Certainly the process of mixing colors can be complicated because most of the time you’re not dealing with a single color but a composite. Your eyes need to tell you what colors are in the mixture so you can “hit the noteâ€.
One way to do this is to realize that you may not know exactly what color you want and how to get it, but you can probably tell what you don’t want. If you mix a color and make a few brushstrokes with it, you need to analyze right away whether it is really the color you want. If it isn’t, go back to your palette and take another shot at it.
To mix colors, you should first have a color wheel in front of you as a guide. You will want to understand that a complementary color is the color across from another color on the wheel. For example, the color green is across from the color red, so red and green are complements of one another.
Whatever color you are mixing, you should be able to decide what definite primary or secondary color you’re dealing with. (Secondary colors are mixtures of the primaries next to each other on the wheel, such as blue plus yellow equals green, the secondary). Usually almost any color, no matter how muted, can be classified as being mostly red, yellow, blue, orange, green, or violet. So you are aiming to modify this main color to get the color you want, either by adding other colors or by adding white, black, or brown. (Yes, you can mix brown. Try mixing all three primaries in different amounts until you hit it.)
To be successful at mixing a color you want, you will need to work mainly in one of two directions. If you want really bright, pure color, don’t use complements in the mixture. But if you’re after a more muted effect, use the color’s complement.  For example, if your green mixture is looking TOO green, put in a little red to tone it down.
There have been entire books written on the fine points of mixing color, but here are some hints to remember: 1. To make a color’s shadow, try mixing the color plus its complementary color. If you use black mixed with a color to get a shadow color, sometimes it works — but you may get a surprise. For example, yellow and black make green, which is not a good shadow color for yellow. But yellow and a dab of purple make a fine shadow for a yellow area. 2. If a color is hard to define, it’s usually made up of all three primary colors.
cc Source: Art-to-Art Palette Journal/Paint Box Section print edition