I found this in another artist's toolkit for her painting style and she was kind enough to post it. You can buy it mixed from her but I mix my own. It's called "Anchor Green" and it's a mix of Chromatic Black and Cad yellow Light. I start with a rough 3-1 mix.. 3 parts yellow to 1 part black. Don't use anything other than Chromatic black. CB is a transparent black made from 3 colors. It won't muddy the color you mix it into like mars black and the like. My based green looks almost black and I add yellow to bring out the green as I work. White will flatten out the green and lighten it. I just LOVE this for foliage as the darks and medium greens.. I like it better than sap green. The same artist also has a awesome grey mix which like a blue /violet that works well for skies and shadows. I'll do a color card on that too in a bit.
Here is a fast and dirty color card