Pretty easy really, instead of mixing with water like normal plaster, just mix in thinned Acrylic paint, colors of your choice. I prefer the types of Acrylic paints purchased at craft stores, but I've seen it done with brown shades of powdered tempera paints as well. Apply with an old brush or spatula. Be careful to make the mixture darker, as it will lighten when it dries. You can expereiment with small amounts until you are comfortable with the mixture.