Green and orange make a shade of brown when mixed. The shade of the brown depends upon how much red and yellow are in the orange and the ratio of orange and green in the mix. Black or white can manipulate the brown's intensity.
Green and orange are secondary colors: both made of two primary colors. A secondary color mixed with a primary color that isn't elemental to it or another secondary color results in brown. The same occurs when mixing a third color, a color between a secondary and primary color, with a primary color that isn't already in the third color.