Duotone is a halftone reproduction of an image using the superimposition of one contrasting colour halftone (traditionally black) over another color halftone. This is most often used to bring out middle tones and highlights of an image. The most common colors used are blue, yellow, browns and reds.
Now due to recent advances in technology, duotones, tritones, and quadtones can be easily created using image manipulation programs.
WHEN TO USE DUOTONE?