A typical black-and-white photograph uses only one color. In a duotone, though, the black-and-white photograph (or other artwork) is reproduced using two colors. Perhaps it’s black and brown, or black and grey, or dark grey and a rusty color. Halftone images are generated for the photograph, one slightly underexposed and one slightly overexposed, and the two are printed one on top of the other. The result can be an incredibly rich, powerful image-much richer and more interesting than the image with one color. The artist/designer has control over the values and percentages of the two different colors. It is also possible to make “tritones” using three different colors, and “quadtones,” using four different colors.
A duotone (or tritone or quadtone) does not refer to the use of spot color for the second color; that is, if you take a photograph and color the lady’s dress pink, that’s not a duotone, even though there are two colors in the image.