Aureolin (sometimes called Cobalt Yellow) is a pigment 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 N. W. Fischer in Breslau and its chemical composition is potassium cobaltinitrite.
Aureolin is rated as permanent in oils but darkens and fades easily 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.