A duotone is a method used to better reproduce the full tonal range of an original fine art black and white photograph in a book or other publication. Fine art black and white photographs are printed on fiber paper, which has either a slightly warm or slightly cool tone. Most fiber prints are toned, generally with sepia or selenium toner, to insure archival permanence. As a result, the full beauty of a photograph is difficult to reproduce using black ink and white paper because the photograph has colors in it that are not black or white.
A duotone is used, mostly in more expensive art books, to come closer to reproducing the full tonal range of a photograph. To accomplish this, two inks are used (hence duotone), black and an additional color, generally either a blue orpurple hue for selenium toned photographs, or a yellow or brown hue for sepia toned photographs. Sometimes, to create an even better representation, three or more colors may be used.