How do you get Olive Green....kelly green and brown?
I would try the brown, green and a little orange. Make saturated balls of paste of the 3 colours and add bit by bit. I find it easier than adding all the colours to the paste.
I haven't tried it with food colors, but with paint, I put yellow with a bit of black and got a real nice olive/sage color. Wouldn't think it from the look of the paint, but when we used it, it was very nice!