This color was reformulated in 2015 (see https://justpaint.org/beauty-and-the-best-wrestling-with-changes-in-williamsburg/). The old formulation was PO73 and PY83. Previous to 2015, Golden found this color to be less lightfastness than its current iteration at ASTM II, and so they improved the formulation. In 2023, Williamsburg undertook additional lightfastness research. 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.