This color is also part of their Landscape Colors series: “An excellent outdoor palette that allows artists to produce the wide variety of hues, shades, and tints encountered in nature.” Also, Williamsburg recently conducted lightfastness tests with various white paints. 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. This blend contains phthalo blue, which displayed some surprising lightfastness behaviors. See https://justpaint.org/astm-lightfastness-testing-for-oil-paints/ for more information.