Scenario: You are designing a piece which will be printed in two ink colors. Since you cannot reproduce full color images (CMYK) with two inks, you must prepare image files for spot color printing by converting them grayscale images and then creating duotones or spot channels.
Grayscale, Monotone, and Duotone
A black and white photograph is called a grayscale image because it contains up to 256 shades of gray when it is printed.
A grayscale image that is printed in any single ink color other than black is called a monotone.
Images that are printed in two, three, or four ink colors are called duotones, tritones and quadtones.
Duotones consist of two images which print on top of one another. Each image is printed in a different ink color. The duotone above is printed first in Pantone Gray and again in Pantone 279.
Duotones contain both colors throughout the entire image. By using a spot channel instead of a duotone, a second color can be applied to selected areas only.