What is a duotone in printing?
A duotone is a grayscale image printed with two inks, usually black and one color, each with its own tone curve, to give a photo more depth or a color tint without full-color printing.
What it means for your file
Photoshop makes them from a grayscale image (Image → Mode → Duotone), which also offers monotones, tritones and quadtones. Each ink is named, often as a Pantone spot color, so the image is saved as a Photoshop PSD or PDF that keeps its inks. On a CMYK-only job, the inks have to be process colors, or the image converted to CMYK, before it prints.
The inks in a duotone are usually spot colors; for the opposite, see process color.
An example
A brochure printed in black and one navy Pantone can still carry rich photos: each is a duotone, with black holding the shadows and detail and the navy adding color through the midtones, so the pictures match the brand. On a four-color job the same look is made with process inks, for example black plus a cyan curve.
A common mistake
Faking a duotone with a color overlay or a blending mode and sending it to a two-ink job. The result is an RGB or CMYK image that has to be split into four inks, not two. Make it in Photoshop’s Duotone mode with ink names that match the job’s swatches exactly, then check the inks in Acrobat’s Output Preview or InDesign’s Separations Preview.
How to fix it
If your printer sent the file back saying “Convert the spot colors to CMYK,” the fix page has the step-by-step way to do it yourself, and a fixed price if you’d rather have it done.
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