Mixing red and green results in a brown or gray color. The exact shade depends on the ratio of the original colors and the amount of pigment present.
Brown and gray are tertiary colors, which means that they are colors that contain all three primary colors. In this case, the primary color red is added to its complement, green, which is composed of the other two primary colors, yellow and blue. Interestingly, though they all contain the same three primary colors, the brown created by this mixture is different than the brown created by mixing yellow and purple or blue and orange.