That adding yellow to black can produce green is a mix most people discover by accident. It may seem improbable, but the combination produces an earthy, dark green. Again, different yellow pigments and different black pigments give different results.
Perylene black is a black pigment (PBk31) that's often labeled Perylene Green because it has a green undertone to it. Use straight from the tube, it's extremely dark, but spread it around or thin it with water/medium and you start to see the green in it. Mix with white and yellow, and it's very evident.
