Photoshop > Image > Mode > Duotone. Duotones originated from halftone printing because presses could achieve better detail by introducing an additional ink color (beyond basic black) to achieve more simulated depth. In Photoshop, the process works much the same way for the same benefits.
Start with an 8-bit grayscale file, or convert it (this command is also found under the Image > Mode menu). From here, you can convert the Image Mode to Duotone. The default Duotone Options panel will display a Monotone setting, with that single color set to black. If you were to change that monotone ink selection to blue, for instance, the entire image would be blue and white. This is actually a great method for re-creating the look of a cyanotype print, but since we want more depth and subtlety in our black-and-white image, switch from Monotone to Duotone and for the second ink choose another color. For simple warming, reds, oranges and yellows are ideal. Cooling works well with blues and cyans. For sepia toning, I usually look in the browns and tans, although some prefer their sepia more red or yellow. It’s all personal preference. I’m fond of Pantone 4525C, as well as the nearby Pantone colors that are slightly more yellow and brown.
Ignore the Overprint Colors checkbox for the moment, as it’s used for tritone and quadtone images in which multiple ink colors are used. That brings us to an interesting multicolor option: split toning. In the chemical darkroom, this meant two toning baths used to impart two separate tones to the image, the result of which often appears richer and with an enhanced illusion of depth. Warm highlights and cool shadows, for instance, are a popular split-toning approach, and this can be emulated with Photoshop’s Duotone mode by choosing a third ink color. Another method is even easier, I think, and that’s found in Photoshop Lightroom.
LIGHTROOM SPLIT TONING
Lightroom > Develop > Split Toning. In Lightroom’s Develop module, convert your selected image to black-and-white. Lightroom’s controls for this are outstanding—much better than simple grayscale conversions. Below the B&W option in the Develop module is an option for Split Toning. Just as it works in the chemical darkroom, this method allows you to impart separate tones to highlights and shadows.
Dealing with highlights first, I prefer to slide the saturation all the way to 100% so I can easily see the color with which I’m working. Clicking on the Color Swatch icon brings up a gradient picker so you can choose the precise highlight tone. In this case, I chose a warm tone at 43 on the scale.
Turning my attention to the shadows, I chose a cooler blue, number 225. The saturation boosted to 100 makes a horribly gaudy split-toned print, but simply dialing back the saturation does wonders for the image. Adjusting the slider allows you to shift the overall balance of these two tones until they make the perfect combination. Since there’s no right or wrong approach, success is in the eye of the beholder. That holds true for digital toning, in general, which is just another reason why it’s such a great technique.