A bit of confusion surrounds this name space. The name Mosaic Gold can refer to a couple of different colors. We rely rather heavily on Artiscreation and Cameo as sources are sparse. On Cameo they note that Mosaic Gold was used in manuscript illumination as an inexpensive gold, and it may have a subtle shine. It seems to have been replaced by brass powder, and one rarely encounters this pigment code. In the comments on Artiacreation, he notes, " The term "mosaic gold" is also used to designate an alloy consisting of 65.3% copper and 34.7% zinc," which is also mentioned on Cameo as a brass alloy.
Artiscreation lists PY38 as Tin Sulphide, which has a number of chemical names in the composition. The top two are, "Tin(IV) Sulfide Berndtite and Tin Sulphide (SnS2)."
We've seen some pigment sellers offer this as Mosaic Gold as well.
On Cameo, it's noted that it refers to a pigment made of Stannic sulphide.
