When mixing color, I break these two rules - sometimes.1. Don't use black in a painting.
2. Don't mix more than three colors together at a time.
The above two rules were taught in classes I took way back when. I see them pop up in art books & on blogs. I have done this when I paint with guache AND in the above pear palette knife acrylic unfinished painting.
Black was mixed in to get the darkest green you see above. There, I admit it. If you are new to painting, just use very small amounts of black at a time. It is a strong color and you won't need much when your are mixing. In other paintings, I have made a lovely mud out of several colors.
NOW, I think these rules are great. They help keep me from over mixing paint. Not using black can give a painting more interest and help to limit your palette.
Am I the only black-user-mud-maker out there?