Wikipedia
Aureolin (sometimes called Cobalt Yellow) is a pigment sparingly used in oil and watercolor painting. Its color index name is PY40 (40th entry on list of yellow pigments). It was first made in 1848 by Nikolaus Wolfgang Fischer in Breslau and its chemical composition is potassium cobaltinitrite. The investigation by Gates gives the exact modern procedures for the preparation of aureolin and also the methods for its identification in paintings.
Aureolin is rated as permanent in some reports but there are other sources which rate it as unstable in oils but pronounce it stable in watercolors. It is a transparent, lightly staining, light valued, intense medium yellow pigment. It is a rather expensive pigment and sold by several manufacturers of oil paints such as Grumbacher, Michael Harding, and Holbein. However, the pigment was never popular as an oil color and is much more widely available as a watercolor from manufacturers such as: Winsor & Newton, Talens Rembrandt, Rowney Artists, Sennelier, Art Spectrum and Daniel Smith.