In Photoshop, duotone refers to monotones, tritones, and quadtones as well as duotones. Monotones are grayscale images printed with a single, non-black ink. Duotones, tritones, and quadtones are grayscale images printed with two, three, and four inks. In these images, colored inks, rather than different shades of gray, are used to reproduce tinted grays.
Duotones increase the tonal range of a grayscale image. Although a grayscale reproduction can display up to 256 levels of gray, a printing press can reproduce only about 50 levels of gray per ink. For this reason, a grayscale image printed with only black ink can look significantly coarser than the same image printed with two, three, or four inks, each individual ink reproducing up to 50 levels of gray.
Sometimes duotones are printed using a black ink and a gray ink—the black for shadows and the gray for midtones and highlights. More frequently, duotones are printed using a colored ink for the highlight color. This technique produces an image with a slight tint and significantly increases the dynamic range of the image. Duotones are ideal for two‑color print jobs with a spot color (such as a PANTONE Color) used for accent.
Because duotones use different color inks to reproduce different gray levels, they are treated in Photoshop as single-channel, 8‑bit, grayscale images. In Duotone mode, you do not have direct access to the individual image channels (as in RGB, CMYK, and Lab modes). Instead, you manipulate the channels through the curves in the Duotone Options dialog box.