One of the least-known color modes in Photoshop, or certainly lesser-known today, is Duotone. This may be partly because it requires converting a file to grayscale first, which means that for color images the very name is grayed-out on the menu. But it has a couple of interesting properties.
The first thing, as I mentioned, is to take an image and convert to grayscale; from there, one can go to Duotone mode. And then, as the one chap said in the movies, the fun begins. Duotone mode allows the user to “tint” the image with between one and four ink colors, rather like “dunking” the image in a dye bath. So what? you might ask.
Although one might want to do brightness and contrast adjustment beforehand, Duotone mode allows some artistic things to happen to an image. For one, it’s possible to print a grayscale-like picture in the four-color process (since the max number of colors one can use is four, they can be cyan, magenta, yellow, and black—CMYK) and make the grayscale-like image a little warmer or cooler by tweaking the balance of the colors in the dialog box.
For another, it’s capable of closely simulating a couple of older photographic printing processes which were considered very beautiful and elegant back in the day. Some illustrated novels, even graphic novels, occasionally still use these kinds of repro art styles.
One, called cyanotype, was used till fairly recently not only to do photo printing but also to create traditional-style blueprints for building and manufacturing. (Since the chemicals involved, ferric ammonium citrate and potassium ferricyanide, are both rather nasty, being able to create the appearance without the potential to poison oneself is certainly a plus!) Even the sub-mode called Monotone can handle this one.
Another, perhaps one of the most beautiful, is sepia toning. A print was washed in a bath that included sepia ink (from the marine animal called the cuttlefish), which both stabilized the image chemically and gave it the lovely golden-brown cast we often associate with antique photographs. Sepia toning helped prevent fading, and gave the image a warmth which a normal silver print would sometimes lack due to its regular color. Duotone mode with all four inks (Quadtone) makes this version easy.
Some newer users of Photoshop love playing with the ability to bend reality—which is cool. But there are a lot more capabilities built into the program. Using it to reproduce some of the beauty of yesteryear doesn’t hurt.