Halftone in the context of "Duotone"

Duotone (sometimes also known as Duplex) is a halftone reproduction of an image using the superimposition of one contrasting color halftone over another color halftone. This is most often used to bring out middle tones and highlights of an image. Traditionally the superimposed contrasting halftone color is black and the most commonly implemented colors are blue, yellow, brown, and red. There are, however, many varieties of color combinations used.