Help в фотошопе по слову duotone:
To convert an image to duotone
1. Convert the image to grayscale by choosing Image > Mode > Grayscale. Only 8‑bit grayscale images can be converted to duotones.
2. Choose Image > Mode > Duotone.
In the Duotone Options dialog box, select Preview to preview the image.
For the Type option, select Monotone, Duotone, Tritone, or Quadtone.
3. Click the color box (the solid square). Then use the color picker or click the Custom button in the color picker to select an ink from the Custom Colors dialog box.
Note: To produce fully saturated colors, specify inks in descending order—darkest at the top, lightest at the bottom.
4. Click the curve box next to the color ink box and adjust the duotone curve for each ink color.
5. Set overprint colors, if necessary.
Click OK.
To apply a duotone effect to only part of an image, convert the duotone image to Multichannel mode—this converts the duotone curves to spot channels. You can then erase part of the spot channel for areas that you want printed as standard grayscale.