Certain combinations of colors will produce the color brown. Not just any combination will work, however.
For example, mixing Red with Orange (the product of Red and Yellow) produces Reddish-orange, not brown. Mixing Blue with Purple (the product of Blue and Red) produces a bluish purple.
If you mix a primary color with the secondary color opposite it on the color wheel, you get brown. Alternately, if you mix all three primary colors, you will also get brown.
For instance, mixing Blue with Orange (which is actually red and yellow) produces brown. Basically, mixing all three primary colors with each other in some fashion will produce brown.
Or a sickly, puke-colored mess.