Fundamentals of Color Printing

While a printed color image may appear to contain thousands of individual colors, it usually consists of just four inks, referred to as process colors: cyan, magenta, yellow, and black (CMYK). The process inks are transparent, so when they are combined on paper, they produce other colors (Figure 2.4). Thus, cyan plus yellow makes green. Cyan plus magenta make violet. Yellow and magenta make red, and yellow and magenta combined with cyan makes an unattractive muddy brown. That's still a fairly small box of crayons. How can you make all the colors you need?

Figure 2.4. The four process inks combine to create much more than just four colors.

In traditional offset printing, the illusion of so many colors is the result of varying sizes of halftone dots, which allow different amounts of the four process colors to interact in a given area. Other printing methods use different ruses to fool the eye into seeing more than four colors, but the concept is the same: Use varying amounts of CMYK to approximate a wide range of colors (Figure 2.5).

Figure 2.5. Varying sizes of halftone dots add up to an optical illusion (section of image greatly enlarged). Note the rosette pattern formed by the dots.

It's important to avoid unsightly patterns, called moiré (Figure 2.6). To see the moiré effect, put one piece of window screen on top of another, and then rotate one piece of screen. It's a challenge to eliminate an obvious pattern. That's why there are time-honored intervals of 30 degrees between the angles of the inks to create the desired rosette pattern (Figure 2.7). Yellow, being the lightest color, falls at a 15-degree angle away from other colors.

Figure 2.6. Incorrect intervals between screen angles can result in a distracting moiré. This can also be caused by a combination of screen angles and image content such as woven fabric.

Figure 2.7. Optimal screen angles add up to form a rosette pattern. Yes, it's a pattern, but it's usually not noticeable.

Screen angle preferences vary between print service providers and may sometimes be chosen to accommodate job content (Table 2.1).

Table 2.1. Screen angle combinations are meant to minimize patterns. Traditionally, they are 30-degrees apart (for example, 45 degrees and 75 degrees). The angle of each color is chosen to minimize interference with the other colors when all inks are combined in the printed piece.

Screen Angle Combinations
C 75°	M 15°	Y 0°	K 45°
C 15°	M 45°	Y 0°	K 75°
C 105°	M 75°	Y 90°	K 15°

Why CMYK not CMYB?

It's easy to imagine that B might be mistaken for blue and thus confused with cyan. But where does the K come from? One theory holds that the K is from key, referring to black being printed first, so other colors can be aligned to its registration marks. Another notion is that the K is lifted from the end of the word black. Whatever the true origin of the mysterious K in the acronym, it's here to stay.

Solving the Moiré Problem: Stochastic Screening

One solution for moiré is to eliminate screen angles entirely by using a printing method without the conventional grid of regularly spaced dots. Stochastic screening, also called FM (frequency modulation) screening, uses a seemingly random distribution of very small dots (Figures 2.8 and 2.9). If you have an inkjet printer, you have a stochastic output device right on your desk. Look at an inkjet print through a magnifying loupe, and you'll see how the scattered arrangement of tiny dots creates an image.

Figure 2.8. Stochastic screening

Figure 2.9. Enlarged detail of stochastic screening

After its first appearance in the 1980s, stochastic screening failed to gain much acceptance due to limitations in plating and proofing systems of the time. But it is experiencing a slight (and cautious) resurgence in the printing industry, thanks to the advent of computer-to-plate (CTP) printing, as well as increased implementation of digital (rather than film-based) proofing systems

Stochastic screening offers some interesting advantages over conventional halftones:

• Reduced chance of moirésince there are no angles, there is almost no chance for an interference pattern to be created.

• Ability to use lower resolution images.

• Ability to print images containing more than four ink colors without screen angle issues.

• Retention of smaller details in images.

• Reduced ink usage.

• Misregistration on press is less obvious.

• Smoother rendition of skin tones.

But stochastic screening has not replaced the old-fashioned halftone dot. In fact, it's used only in a minority of printing. There are some challenges to using FM screening:

• The need for extreme cleanliness in plating: Dust may be bigger than a stochastic dot.

• Modifications to RIPs can be expensive ($15,000$25,000).

• Slightly increased RIP processing time.

• On-press dot gain is higher than with conventional screening.

• The possibility of visible graininess in large highlight areas.

Consult your print service provider to determine whether stochastic screening is something they offer and whether it might be appropriate for your job. Be prepared for the possibility of increased job cost because of special handling.

Screen Values: Recipes for Color

When you need to describe a combination of cyan, magenta, yellow, and black that will print a particular color, such as the dusky blue below (Figure 2.10), the recipe is written in this format: C75M50Y25K0. Think of halftone dots as occupying a square grid, each in its own square of the grid. The numbers signify a percentage of that square that will be filled. If the square is full, it's 100 percent. If half the area of the square is filled, it's 50 percent, and so on.

Figure 2.10. Dusty blue is 75 percent cyan, 50 percent magenta, and 25 percent yellow.

Limitations of CMYK

While an extensive range of colors can be rendered with various combinations of cyan, magenta, yellow, and black ink, there's still a limit to what CMYK can create. The human eye can see a huge range of colorslarger than even the large gamut of a computer monitor. But the total gamut of the process inks is considerably smaller than the human eye can see, or even the range that the monitor can display. Consequently, images that are quite vibrant on your monitor may print disappointingly dull. It's not because your print service provider is incompetent. It's because of the limitations of the printing-ink spectrum. In Figure 2.11, the large, colorful toe is an approximation of the range of colors perceived by the human eye. The solid triangular line corresponds to the range of colors that can be displayed on an RGB (red-green-blue) computer monitor. The much smaller dotted shape indicates the approximate gamut of CMYK inks. Note that the CMYK blob, while rather constricted, does not fall entirely within the RGB gamut. Some colorsbright yellows and cyan shadesfall outside the range that can be displayed faithfully on a monitor. Even a finely tuned and color-managed monitor has its limitations.

Figure 2.11. A rough comparison of monitor and CMYK gamut to the range of visible light.

This is not intended to plunge you into despondency over the limitations of the printing process. After all, disappointment is all about expectations. If your expectations are unrealistic, you're bound to be disappointed. But if your expectations are realistic, you can be prepared for the limitations of CMYK. And, equally importantly, you'll know when you need to step outside the world of cyan, magenta, yellow, and black to get what you want.

Spot Colors

A spot color is used when it is necessary to print colors that fall outside the range of CMYK inks. While it's fairly intuitive that inks such as fluorescent colors and metallics can't be faithfully imitated by process colors, there are also some rather common colors that fall outside the CMYK universe, such as bright orange and navy blue.

You're probably familiar with the Color Formula Guides from Pantone® Inc. The terms spot color and Pantone are often used interchangeably, although that's not strictly correct. A resource such as the Pantone Matching System swatchbook is actually a recipe book for print service providers. It provides ink-mixing formulas for creating over 1,000 standard colors, many of which cannot be accurately rendered in process colors. It's forgivable that the name Pantone has become synonymous with spot color, since the fan-like Pantone swatchbooks are probably the most common reference for specifying color. But there are other spot-color resources, such as the Toyo Color Finder from Toyo Ink, and the DIC guide from Dainippon Ink and Chemicals, Inc., (used predominantly in Japan). And there are Pantone swatchbooks that don't depict spot colors, such as the Pantone 4-Color Process Guide, which contains only colors created by combinations of cyan, magenta, yellow, and black.

While adding a spot color to a four-color job may slightly increase the cost of printing because of the need to create an additional plate and purchase additional ink, it ensures that important colors will print as desired. Using a spot color can also eliminate problems caused by slight misregistration on press. Consider a job containing elements in a burgundy color consisting of a process-color build of C10-M100-Y35-K50. Even the most conscientious pressman can find it challenging to keep fine elements such as small type and narrow rules in register across a large press sheet. It can also be difficult to keep the balance of four inks consistent from one part of the paper to another or from one press sheet to another in a long, multipage job. Any variation will result in color shifts, which would be especially noticeable in facing pages. Replacing the process build with a single ink, such as Pantone 209, simplifies both registration and color-consistency issues. The small increase in printing costs (as opposed to a four-color job) might be justified by the improved outcome.

CV, CVC, CVU, M, C, U: Many Acronyms, Just One Ink

It's time to dispel some urban myths about spot-color designations. The terms Coated and Uncoated refer to paper, not ink. Pantone 185C is Pantone 185U is Pantone 185M (apologies to Gertrude Stein). The C represents coated paper, U signifies uncoated stock, and M indicates matte paper, whose surface texture falls between that of coated and uncoated. These designations are primarily intended to keep you oriented as you view color on your computer monitor. For example, you may notice that a U version of a Pantone color looks a bit less saturated compared to the C version. It's just an attempt to mimic ink behavior on different stocks. In the olden days, CVU meant computer video uncoated, and CVC meant computer video coated. But recent DTP software has simplified this to U and C, and added the enlightened M for matte.

Process Color Guides

Since its inception, the TRUMATCH Colorfinder from TRUMATCH, Inc., has provided only CMYK builds.

The Pantone Corporation also provides several CMYK-based color specifiers:

PC: Solid to Process. Uses the same familiar Pantone numbers but indicates process builds.

EC: Solid to Process (Euro). Same concept as PC, but with slightly different CMYK equivalents.

DS: Process Specifier. Does not use ancestral Pantone numbers, which may help avoid confusion.

Approximating Spot Colors with Process

It's a widespread practice to pick colors from a swatchbook such as the Pantone Color Formula Guide, even for jobs that are intended to print as process. Just because everyone does it doesn't mean it's right. (Sorry. That sounds like your mother.)

The problem with this approach (as with so many things your mother warned you about), is that it can lead to disappointment. Remember that the purpose of spot colors is to render colors that fall outside the range of CMYK. Understandably, process approximations of spot colors are often unsatisfactory.

For example, a CMYK translation of a dark blue such as Pantone 286 can become a purplish blue (Figure 2.12). It's unfortunate, but this is as close as a combination of cyan, magenta, yellow, and black can get to the navy blue that should be used for the Brand X logo. As long as you know to expect this color approximation, you aren't shocked by the printed piece. But the president of Brand X will certainly be disappointed.

Figure 2.12. The Brand X logo is supposed to print in Pantone 286, a navy blue. But approximating that color with CMYK results in an unsightly purple. If you have a Pantone swatch book, compare the real Pantone 286 swatch with the CMYK version at left.

In the interest of realism on process jobs, consider selecting colors from a purely CMYK-based swatchbook instead, such as the TRUMATCH Colorfinder or one of the Pantone process guides. If you want a single-source swatchbook showing Pantone spot-color formulas next to their closest process equivalents, the Pantone Color Bridge™ provides helpful, side-by-side swatches.