Duotone

Duotone is a halftone reproduction of an image using the superimposition of one contrasty color 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.

Thanks in part to Photoshop, duotones, tritones, and quadtones can be easily created.