This is a super mysterious and weirdly fascinating color is also known as Green Gold. It's magic is not its masstone but rather what it does in mixes-- it is concentrated sort of like how yellow food dye often doesn't look yellow, but a drop of it in white brings out bright color. This transparent color is like the essence of deep yellow-greens, and it is actually necessary to expand the gamut. In masstone, it's actually quite green but it takes on more of a yellow in tints. PY129 shines in the low notes of the yellow-greens and allows some of the highest chroma low-lightness greens in oil paints.
Toxicity is listed as B so handle with care. It has some things in common with PY150, which is like a warmer counterpart.
Unfortunately in the recent Williamsburg lightfastness tests it displayed more reactivity than we would have hoped depending on the mixing white. In that round of testing, it looks like it did not respond well to a handful of things such as Lithopone, Safflower Oil, Zinc, or Flake White. Depending on the brand of Flake/Cremnitz Lead White, PY129 might not fare well. This was interesting news because PY129 is such a great pigment for a hard-to-replace area of the gamut and for us it may be worth it to use it in the ways wherein it performed well. See Golden's Testing for more.
