Often described as Hansa Yellow Medium. Somewhat rare in oil paints but more frequently found in watercolor. This is one of many Hansa yellows, and like other hansas the lightfastness may vary from brand to brand even among paints that share the same pigment code (the Hansa that brought this to our attention was PY3- see notes on that pigment for more).
Maimeri ascribes the name Primary Yellow to this bright yellow color. It seems to have decent lightfastness among the better brands, but even then it is only at ASTM II. This color fades a bit in tints.
There is some discrepancy in the opacity of this pigment with some sources describing it as transparent and others as opaque. This article from the coatings world describes it as having "useful opacity".
This pigment may metabolize to a potential carcinogen. This could cause methemoglobinemia, see Monona Rossol's writing for more.
