10. Spot Colors and Duotones: Special Inks for Special Projects
The fundamental problem with most printing presses is that they can only print one color at a time. It's like pixels on a black-and-white screen: The color is either on or off. You may have a gloriously rich full-color image on screen, but you've got to be mighty clever to get that image out the back side of a printing press, and no matter what you do, there will be trade-offs involved.
There are two methods for printing color on a press: spot color and process color. Both can give you a wide variety of colors. But they are hardly interchangeable.
Process color. As we've noted throughout the book, process color is the method of printing a wide range of colors by overlapping halftones (tints) of only four colors: cyan, magenta, yellow, and black. The colors themselves do not mix on paper. Rather, the eye blends these colors together so that ultimately you see the color you're supposed to.
Spot color. If you are printing only a small number of colors (three or fewer), you probably want to use spot colors. The idea behind spot color is that the printing ink is just the right color you want. With spot color, for example, if you want some type colored teal blue, you print it on a plate (often called an overlay) that is separate from the black plate. Your commercial printer prints that type using a teal-blue inkprobably a PMS inkand uses black for the rest of the job.
Because process colors simply cannot simulate some colorslike deep blues, and metallics like goldspot colors are also used as bump plates and varnishes that print alongside or on top of process-color images. For instance, a picture of a fancy car might be printed with the four process colors, plus a spot red to highlight ("bump up") some areas of the car, plus a varnish over the image to make it glossy.
This is relatively easy to print on a six-color press. The hard part has always been building the spot color and varnish plates.