Color Mixing for Somber Hues oil painting

Oil painting in process, 20" x 20", o/c

The plan for this painting is to use deeper, somber hues contrasting with brighter, more clear hues. Although I have always been a colorist, I am learning more and more about color with each project that I take on. The trick about mixing somber hues is to mix only two colors if possible in order to avoid the dreaded muddy colors. The rich bue-green on the upper right is a mix of Cerulean and Terra Rosa. It is super beautiful. Grays are something that I am enjoying more and more, and generally I mix a warm gray using Yellow Ochre and French Ultramarine with white. The black on the upper left is a mix of French Ultramarine and Terra Rosa. I have never used black out of the tube. Why? From my landscape painting training (School of Light and Color), it just never fit into my palette. However, when I want a very dark  "color" I mix my own. I mixed a little Yellow Ochre into the white for the white square, (the 3rd painted square down on the right side) as I think I will prefer it to be a bit warmer with the color composition.