Brown can be made by mixing orange and black or by mixing green and red. The resulting color is not likely to be the warm, pure brown most associated with the color's name, but the result can be lightened and the shade changed by adding a bit of white as well.
Brown is considered a tertiary color because it can be made by mixing a primary color (red) with a secondary color (green). The three primary colors are red, yellow and blue, and the three secondary colors are orange, green and purple. Because neither black nor white is on the color wheel, the method of mixing black and orange may be less desirable.