Four-color process printing is an amazing procedure. Few people outside of the printing industry fully realize either the complexity or the different processes available for converting a color photograph for use in a printed medium. Successful four-color process printing requires a thorough understanding of the nature of light, color theory, the composition and function of different inks, and the way our brain perceives color.
In four-color process printing, inks are used together to create new colors. Because process inks act much in the same way as filters, subjects containing several different colors or gradations of colors can be reproduced using just three colors of ink: yellow, magenta (bluish-red), and cyan (blue-green). Process yellow absorbs only blue light, magenta absorbs only green light, and cyan absorbs only red light. When yellow is printed on top of magenta, the result is a shade of red. Yellow printed on top of cyan results in a shade of green. In theory, when yellow, megenta, and cyan are printed on top of one another, black should be the result. In reality the result is a brownish color, due to the nature of pigments. To help compensate for this, black is added as the fourth color in four-color process printing. Black also creates added depth and definition to the reproduction.
Look at the photograph above, you can differentiate hundreds of different colors. However, we printed it using only four inks. First, we took the original subject and, using a color scanner, produced a four-color separation. The color scanner evaluated the colors in the original photograph and electronically determined how much yellow, magenta, cyan, and black it would take to approximate each color in the photograph. The scanner then created the four different component films of the four-color separation, one for each different printing ink that was used. Eventually, through a complex series of procedures, printing plates were produced from this four-color separation. These plates were the image carriers used on the printing press that enabled the press to transfer ink to paper.
This transfer resulted in an image consisting of hundreds of thousands of different-sized dots, just like in a black-and-white halftone, except that now instead of just one black-and-white halftone, there were four colors of ink laid on top of one another. If you look at the photograph through a magnifying glass, you can see some of the dots are printed on top of one another, some printed right next to each other, and some are just close together. The viewer's mind is constantly blending the dots approximating the colors found in the original subject.