This color is part of their Signature Color series: “Throughout Williamsburg's history, exploration and experimentation have led to the development of several unique colors. The Signature Colors set contains 8 colors representative of these discoveries.” Also, while not a commercial set, this color is mentioned as part of Williamsburg’s six Chromatic Darks. From their publication, Just Paint they explain, “These colors are all blends, containing a natural iron oxide or Ivory Black and one or more strongly colored pigments to give them their chromatic punch. When painted from the tube, they visually resemble a colorful black, but when scumbled, tinted or mixed, these darks start to show their true color!” They go on to share about Courbet Green specifically, “Calcined Yellow Ochre, Prussian Blue and Permanent Yellow Deep combine to make Courbet Green a sophisticated dark green. In masstone and undertone it appears warm throughout, but changes temperature when mixed with white. Courbet Green tints to a minty cool, bluish green. It is similar to Turkey Umber but more extreme in every way. The masstone is darker, the tinting strength is stronger and the range from warm, glowing undertone to cool tint is much more pronounced. It is Semi-Opaque, very fine and fast-drying.” -Just Paint 2020. As a mixture, this color does not appear to have been specifically mentioned, however it is worth noting that Golden published groundbreaking lightfastness testing in various white colors. The choice of mixing white may affect the performance of certain pigments. See https://justpaint.org/astm-lightfastness-testing-for-oil-paints/ for more information.