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The Color Story

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By Nancy Plowman
President, Nancy Plowman Associates, Inc.

Are these statements true or false?

  • Printing is a subtractive process.
  • A printing press can only subtract color from white paper, not add color to it.

Have you ever stopped to think about the "process" of process printing? Can you explain how such beautiful color could be produced using only four ink colors? The answers can be found in the color story.

The color story is an explanation of how the printer can use the four process colors, black, cyan, magenta, and yellow, to reproduce the entire color spectrum. Understanding the color story is important when you are formulating your inks for proper wet trapping in multicolor printing. If the inks do not trap properly, color will be off and time will be wasted during makeready in attempting to match a proof. Here's how it goes:

White light can be passed through a prism and broken up into individual bands of color (figure 1). A rainbow, for instance, is formed when light bends around the water in the air. Although many colors are visible, the three primary colors of white light are RED light, GREEN light, and BLUE light.

Red, green, and blue light are additive colors, because when they are added together, the result is white light.

But let's take a look a what happens as we begin to remove the primary color from white light. If we remove the RED light, then two colors of light that remain are GREEN and BLUE. When these two colors are added together, the result is a color called CYAN. Cyan is the "blue" ink used in process printing. Cyan is a SUBTRACTIVE color because when it is printed to white paper it filters out, or subtracts, the RED light reflectance from the paper (figure 2).

If we now remove the GREEN light from white light, the two colors that remain are RED and BLUE. When these two colors are added together, the result is a color called MAGENTA. Magenta is the "red" ink used in process printing Magenta is also a SUBTRACTIVE color, since when it is printed to white paper it filters out the GREEN light reflectance coming from the paper (figure 3).

And finally, if we now remove the BLUE light from white light, the two colors that remain are RED and GREEN. When these two colors are added together, the result is a color called YELLOW, also a SUBTRACTIVE color since when it is printed to white paper, it filters out the BLUE light reflectance coming from the paper (figure 4).

The four-color printing process uses these three colors plus black to create all the various colors in a job. By varying the size of the printed dot, and trapping one ink over the other, just about any color can be reproduced.

In order to produce a three- or four-color screen tint that is consistent throughout a run, register and dot size have to remain virtually perfect. For increased consistency and productivity, the desired shade of color is often run as a special color, where the an ink is mixed to match. This is particularly true when a fifth printing unit is available.

Let's take a quiz. Look at the solids in figure 5 that have been printed onto white paper. Can you determine why Area A is red? It's RED because:

  • Area A has been reproduced by printing yellow and magenta on white paper.
  • White paper contains red, green, and blue reflectance.
  • The magenta ink film filters out (subtracts) the green reflectance from the paper.
  • The yellow ink film filters out (subtracts) the blue reflectance from the paper.
  • Therefore, if the white paper contains red, green, and blue reflectance, and we have subtracted the green and blue, the only color that can reflect from the white paper is red.

Follow the same logic to complete the following two examples:

In figure 5, area B is BLUE, because:

  • Area B has been reproduced by printing cyan and magenta on white paper.
  • White paper contains red, green, and blue reflectance.
  • The cyan ink film filters out (subtracts) the ------ reflectance from the paper. · The magenta ink film filters out (subtracts) the -----reflectance from the paper. · Therefore, if the white paper contains red, green, and blue reflectance, and we have subtracted the ------ and ------, then the only color that can reflect from the white paper is blue.

Area C is GREEN, because:

  • Area C has been reproduced by printing cyan and yellow on white paper.
  • White paper contains red, green, and blue reflectance.
  • The cyan ink film filters out (subtracts) the ------ reflectance from the paper.
  • The yellow ink film filters out (subtracts) the --- reflectance from the paper
  • Therefore, if the white paper contains red, green, and blue reflectance, and we have subtracted the ---- and ------, then the only color that can reflect from the white paper is green.

In the center of Figure 5, there is an area where all three ink films overlap on the white paper. This area is BLACK, since the three ink filters subtract our all of the three primary colors of light coming from the white paper: red, green, and blue. The absence of light is BLACK.

Do note that since ink films are not perfect filters and cannot filter out 100% of the light coming from the paper, a fourth color--black is run to increase density and contrast. It is also used for black-and-white halftones.

Printing halftone dots onto paper is the same as printing solids but on a smaller scale. And each dot subtracts color from the white paper. The halftone dots must trap properly on top of one another in wet on wet printing in a multicolor press. This is called wet trapping. If the dots do not overprint one another, color will be off, and the proof cannot be matched.

Printing is a subtractive process. A printing press can only subtract color from white paper, not add color to it. BOTH STATEMENTS ARE TRUE.

Having an understanding of the subtractive printing process can help the printer when formulating inks that will be compatible with the printing system. It will also be an aid in troubleshooting problems on press when color matching to a proof becomes difficult.

Nancy Plowman Associates, Inc. is a full service graphic arts testing laboratory located in Burlington, Massachusetts.
Telephone 781-272-7410
e-mail to:nancy_plowman@npatest.com

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