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.
Modern use 
Duotone color mode in Adobe's Photoshop uses an imaging process that computes the highlights and middle tones in a black and white image then allows the user to choose any color ink as the second color.
Fake Duotone 
A fake duotone, or duograph, is done by printing a single color with a one color halftone over it. This process is generally not preferred over a regular duotone as it loses much of the contrast of the image.
See also 
- Pipes, Alan. Production For Graphic Designers 2nd Edition, Page 86: Prentice Hall Inc 1997
- Luminous Landscape Duotone Article
- LandscapePhoto.us Article on B/W and toning
- Creative Pro Article
- Working with Duotone in Photoshop
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