A hardener composition for use as a component of a two component polyurethane coating composition, comprising a polyisocyanate, a catalyst for promoting reaction of the polyisocyanate with a reactive hydrogen compound, and one or more antioxidants selected from: a first antioxidant comprising one or more of sterically hindered phenol antioxidants and secondary aryl amine antioxidants, and a second antioxidant comprising one or more organophosphite antioxidant, exhibits improved color stability.