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.

Convert an image to duotone

Modify the duotone curve for a given ink

In a duotone image, each ink has a separate curve that specifies how the color is distributed across the shadows and highlights. This curve maps each grayscale value in the original image to a specific ink percentage.

Specifying overprint colors

Overprint colors are two unscreened inks printed on top of each other. For example, when you print a cyan ink over a yellow ink, the resulting overprint color is green. The order in which inks are printed, as well as variations in the inks and paper, can significantly affect the final results.

To predict how colors will look when printed, use a printed sample of the overprinted inks and adjust your screen display accordingly. Keep in mind that this adjustment affects only how the overprint colors appear on-screen, not when printed. Before adjusting these colors, make sure to calibrate your monitor.

Adjust the display of overprint colors

Saving and loading duotone settings

Use the Save button in the Duotone Options dialog box to save a set of duotone curves, ink settings, and overprint colors. Use the Load button to load a set of duotone curves, ink settings, and overprint colors. You can then apply these settings to other grayscale images.

Photoshop includes several sample sets of duotone, tritone, and quadtone curves. These sets include some commonly used curves and colors. Use these sets as starting points when you create your own combinations.

View the individual colors of a duotone image

Because duotones are single-channel images, your adjustments to individual printing inks are displayed as part of the final composite image. In some cases, you may want to view the individual “printing plates” to see how the individual colors will separate when printed (as you can with CMYK images).

Printing duotones

When creating duotones, keep in mind that both the order in which the inks are printed and the screen angles you use have a significant effect on the final output. (If needed, change the halftone screen angles on the printer’s RIP.)

You do not have to convert duotone images to CMYK to print separations—simply choose Separations from the Profile pop‑up menu in the Color Management section of the Print dialog box (for setting printer options). Converting to CMYK mode converts any custom colors to their CMYK equivalents.

Exporting duotone images to other applications

To export a duotone image to a page-layout application, you must first save the image in EPS or PDF format. (However, if the image contains spot channels, convert it to Multichannel mode and save it in DCS 2.0 format.) Remember to name custom colors using the appropriate suffix so that the importing application can recognize them. Otherwise, the application may not print the colors correctly, or it may not print the image at all.