Toluidine red

Toluidine red is an organic compound with the formula C10H6(OH)(N2C6H3(NO2)CH3). A dark red solid, the compound is classified as a azo dye consisting of a 2-naphthol group linked to a 2-nitro-4-methylphenyl substituent. Toluidine red is a traditional pigment, found in oil paints. Although once popular, it suffers as a pigment owing to "insufficient lightfastness and bleeding when incorporated into a paint system."

Safety
It is classified as carcinogenic, a property that it shares with many azo dyes.