This color is not part of a commercial set, however it 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!” In regards to Indigo specifically: “Raw Umber and Prussian Blue combine to make Indigo a deep greenish blue with intense tinting strength. Similar to the relationship between Courbet Green and Turkey Umber, Indigo is Payne’s Gray’s intensely chromatic counterpart. Indigo has no relationship to the natural, organic dye derived from the fermented indigo plant, except for its potent ability to stain and render things blue. This is a tempered version of Prussian Blue, making it a little easier to shift mixtures without fully taking them over, as can happen when using straight Prussian. It is Semi-Opaque, fine, and very 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.