A coupler is a chemical used in colour photography, either incorporated into the film's emulsion layers or added to the developer. During colour development, couplers react with oxidation products of exposed silver halides to form visible dyes, creating the final colour image. Each of the three layers in colour film-red, green, and blue sensitive-contains its own coupler to produce the corresponding cyan, magenta, or yellow dye. Couplers are essential for accurate colour reproduction and the formation of stable, vibrant photographic images.