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.
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.
3 Select Preview to view the effects of the duotone settings on the image.
4 To specify the type of image, select Type: Duotone.
5 Specify the ink colors, click the color box (the solid square) for each ink.
6. Use the duotone curves to adjust the tonal range of each ink.
7 Click OK.
8. Save the image using the Photoshop (.psd) file format.
Using the duotone curves
1 Click the curve box next to the ink color box.
A. Highlights B. Midtones C. Shadow
The default duotone curve, a straight diagonal line across the grid, indicates that you are mapping the current grayscale value of every pixel to the same percentage value of the printing ink. At this setting, a 50% midtone pixel prints with a 50% dot of the ink, a 100% shadow with a 100% dot of the ink, and so on.
2 Adjust the duotone curve for each ink by dragging a point on the graph or by entering values for the different ink percentages.