Mixing colors can be a frustrating experience for beginning artists. If I mix blue with red I should get purple, right? But it doesn't always work that way. The beautiful purple I was looking for, looks brown and muddy. What did I do wrong?
I was fortunate enough in school to have a really great painting professor who taught us so much about color: how to mix it, how different colors respond next to each other, and how to keep your colors vibrant.
If you don't already have one, it's a great idea to get yourself a basic color wheel. Mixing complimentary colors (those on opposite sides of the color wheel) create different kinds of browns, whereas mixing adjacent colors (those close to each other on the color wheel) will give you a color that keeps its vibrancy and the resulting color stays on that side of the wheel.
However, if I mix a red that has a small amount of yellow in it with a blue that has some green in it, I'll end up with mud! Why? It's because red and green are complimentary colors, as are blue and orange (from the red with the yellow in it.)
For this reason, I buy the most pure reds, yellows and blues I can get. Liquitex paints have a cool scale on every tube - the asterisk shows you just how close to pure green the color is. It clearly shows you if your green is more towards yellow or towards blue. This really helps for more predictable mixing, and I'm surprised other companies don't do it.
Knowing about how complimentary colors mix is useful in knowing what to avoid when you want a more vibrant color, but it is also useful when you want to tone down a color. When I have a red that is too vibrant, I'll add a little green to it to make it more muted.
Another thing I see beginning artists do is use white to lighten color and black to darken them. In my opinion, adding white and black kill the color! My painting professor wouldn't even let us have black and white on our palettes. If you want a light green, try adding some yellow instead. Or darken your red with a little blue or green.
Lastly, you get what you pay for when buying paint. Cheap paints have a lot of filler and not much pigment. They also have a lot of white and black in them. They dry very flat and have little to no sheen, and it's nearly impossible to mix decent colors. The more expensive paints have a higher pigment load and much less filler. They're much more predictable when mixing, and the color dries rich, and the heavy bodied paints keep their peaks when dry.
With a little bit of knowledge and a lot of practice, you can make your colors do just what you want them to. Have fun with it!