An aqueous, homogeneous, single-part color developing concentrate comprises a color developing agent in free base form and an organic antioxidant. The concentrate also includes a water-miscible or water-soluble organic co-solvent and a quaternary ammonium salt to enhance its stability. The concentrate can be used to make a working strength processing solution, or it can be used as a replenishing composition with proper dilution, and is particularly useful for processing color negative or color reversal photographic silver halide films.