Aureolin (sometimes called Cobalt Yellow) is a pigment used in oil and watercolour painting. Its colour 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.