For some reason, creating a duotone (a photo that uses just two colors) in Photoshop is immeasurably more complex than creating one with four colors (as in CMYK). You definitely have to jump through a few hoops to get your duotones to look good and separate properly; but the depth added by combining a second, third, or fourth color with a grayscale photo is awfully hard to beat.
Open the photo that you want to convert to a duotone. If you’re starting with a color photo, you have to convert to grayscale first by going under the Image menu, under Mode, and choosing Grayscale.
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