duotone

Duotone is an image produced using two colors. In printing, it typically refers to reproducing a grayscale image with two ink colors, usually black plus one secondary color. In digital imaging, a duotone assigns two tones to a grayscale image to create a two-color look, often for aesthetic or legibility reasons.

The term originated with two-color printing processes in the late 19th and early 20th centuries, when newspapers and magazines used two inks to save cost while increasing tonal range. Early duotones often used black and blue, brown, or sepia.

Traditional methods involve two ink plates or rollers on a press, with halftone screenings to reproduce gray tones and careful control of dot size and tilt to adjust contrast. Digital approaches convert an image to grayscale and then map light and dark ranges to two chosen colors, often using curve adjustments, gradient maps, or selective colorization of shadows and highlights.

Applications span graphic design, posters, album art, and branding, as well as photographic aesthetics to convey mood. In web and user interface design, two-tone icons and imagery are common, while in film and video, duotone grading is used to achieve a distinctive aesthetic. Printing on colored stock is another practical avenue for achieving the effect.

Considerations include accessibility and legibility, since color choice affects contrast. Limitations involve reduced color information and potential loss of detail compared with full-color imagery. Variants and related approaches include bichrome and tritone processes, which use two or three colors respectively.