A duotone is a form of printing that uses two colors of ink, one of which is often black. Duotones were developed in the printing process as a method of expanding a grayscale image's tonal range, allowing reproduction of delicate colors. In Photoshop, you can use more than 150 preset dutones, tritones (black ink and two other colors), and
Make a selection in the dialog window from the Duotone, Tritone, or Quadtone menu, and then pick the specific colors to make up your new image.
