Violet Oxide or Burnt Sienna
Even though violet oxide and burnt sienna look similar on the palette, burnt sienna gives an almost pinkish tone when mixed with white, while violet oxide produces a nearly neutral color with overtones of violet and grey. I mix it with other colors to accurately paint winter grasses, rocks, gravel, or sand.
I also like to mix it with cobalt blue to make a beautiful grey, or with Hooker's green to make a deep black.