Cobalt yellow (commonly called aureolin) is a potassium cobalt nitrite. The compound was discovered in 1831 but not introduced as a pigment until after 1851. It is an intense yellow that has high tinting strength and low opacity, popular as a watercolor. In watercolors, cobalt yellow is a transparent, lightly staining, intense yellow.
Note: Some separation of pigment and medium may occur in Rublev Colours Watercolors and is a natural process when no stabilizers are added to paint to prevent this from occurring.