Photographic print toning
In photography, toning is a method of altering the color of monochrome photography. In analog photography, it is a chemical process carried out on metal salt-based prints, such as silver prints, iron-based prints, or platinum or palladium prints. This darkroom process cannot be performed with a color photograph. The effects of this process can be emulated with software in digital photography. Sepia toning, the most familiar such process, is considered a form of black-and-white or monochrome photography.
Read on Wikipedia