6 . 3 Color
"We believe in color. Blood, guts, and color." -Dr. Steve Erde (developer of a hypertext system for pathology)
Color can be used in an almost infinite number of ways to enhance a document. While this topic is largely beyond the scope of this book, a brief overview of commonly used color concepts is useful. The addition of color to a document can significantly increase the production costs as well, so it really does pay to be careful with color.(6)
The specification of color to a printer is a difficult issue. The statement "Oh...make it a little redder and put just a touch of blue in it" is not going to suffice as a color specification. The color you see on a display screen will probably not be the color printed, unless you pay careful attention to the particular representation of colorthe color modelused both by the display system and the printer. Of course, the printing industry has developed a number of useful models, and you don't need a degree in color science to print color. However, as a little background, let's examine some of the fundamental color models.
A color model is a particular representation of color, that is useful for at least one particular application. The spectrum of light that we call color can be characterized in may ways. Wavelengths, photons, and obscure mathematical equations can characterize the physics of light. Several color models permit the precise specification of color. Each model is a different way of representing, characterizing, and categorizing color.
6 . 3 . 1 Pure Color Models
The Munsell and CIE models described in this section were invented independent of a particular application, such as printing or lighting. These models describe color for color's sake. They provide a useful language in which to specify color, independent of the application.
MUNSELL
Created by Albert Munsell, the Munsell color system is accepted by many standards and professional organizations concerned with color. (See color plate 1a.) The system describes color using three variables: hue, saturation, and value. Value is sometimes called brightness. In the threedimensional space defined by this system, the central axis represents value, hues are organized around the axis, and saturation increases away from the axis. Color can be precisely specified with a notation such as 10R 6/4, where 10R designates the hue (R is the abbreviation for red), 6 is the lightness, and 4 is the saturation. The numbers all range in a scale from 1 to 10.
One computerbased utility that uses a variation of the Munsell color space is Apple's Macintosh Color Picker. It presents the user with a twodimensional slice through this color space from which to pick a color.
In contrast to Apple's Color Picker is Adobe's Color Picker used in PhotoShop. It is oriented for fine control of saturation and brightness within a constant hue. In addition, it allows the entry of exact percentages of CMYK (Cyan Magenta Yellow blacK) colors.
CIE
In 1931, an international group of experts called the Commission Internationale d'Eclairage (CIE) developed a mathematical color model. The premise used by the CIE is that color is the combination of three things: a light source, an object, and an observer. The CIE tightly controlled each of these variables in an experiment that produced the measurements for the system. "This system is based on the fact that three colored lights (primaries) can be mixed to match any given color. In 1931 the color matching data obtained for the visible spectrum from a set of observers under rigorously controlled conditions were averaged and adopted as the color matching functions of the normal human eye by the CIE."(7) This diagram and the associated systems that use it mathematically plot a color's wavelength and purity. An x,y coordinate system is overlaid with the diagram to provide a precise means of specifying a color. For example, 0.5x, 0.35y is tomato red.
Some electronic publishing systems, such as Adobe's PhotoShop, allow the user to specify exact x,y coordinates corresponding to a color in the CIE system. In the book Color for the Electronic Age, Watson-Guptill, 1990, Jan White aptly sums up the importance of the CIE system:
The value of the CIE system lies in its permanence. Color samples, no matter how carefully produced and preserved, tend to fade over time. The precise CIE numbers, however, can be used as controls in the preparation of replacement swatches.
While the pure color models provide an applicationindependent view of color, they are not particularly convenient for computer displays and printing. Let's examine some of the more applicationoriented color models.
6 . 3 . 2 Computer Graphic Models
In the computer graphics world, the most common color representation is called RGB (red, green, blue). Images are represented in terms of their red, green, and blue components. In addition, display screens (the video tubes) have active areas (phosphors) that emit red, green, and blue light. Combinations of red, green, and blue dots can produce simulations of continuoustone color images. The red, green, and blue dots are so small that the eye and brain blend these discrete dots into a continuous tone. A particular mix of the red, green, and blue components of a screen area corresponds directly to the perceived color of that area.
Another color representation often used in computer graphics is the hue, saturation, and brightness (HSB) model. Because brightness is often called value, the term HSV is also used. Colloquially speaking, hue is the kind of color. For example, red as opposed to blue or yellow. Saturation is the depth or intensity of colorlight red as opposed to dark red. Brightness is the overall amount or lightness of a color.
According to the ASTM Standards on Color and Appearance Measurement, 2nd Ed., 1987, the definitions of these variable are:
Hue - The attribute of color perception by means of which a color is judged to be red, orange, yellow, green, blue, purple, or intermediate between adjacent pairs of these considered in a closed ring (redpurple being an adjacent pair).
Saturation - Attribute of a visual sensation according to which an area appears to exhibit more or less chromatic color, judged in proportion to its lightness or brightness.
Lightness - (1) The attribute of color perception by which a nonself-luminous body is judged to reflect more or less light. (2) The attribute by which a perceived color is judged to be equivalent to one of a series of grays ranging from black to white.
Another method used to display color in computer graphics is dithering. Dithering is not a color model but a particular way of using color. Dither patterns are used to increase the perceived amount of colors on a screen at one time. While a display may be capable of displaying only 256 distinct colors, dithering can be used to raise the perceived number of colors into the thousands. A good example of the use of color dithering is in Apple's Dither Picker. Documentation accompanying the Dither Picker states:
In order to avoid highlighting problems and other unpleasant color look-up table conflicts, Apple recommends strongly that developers create icons from a palette of 34 colors. The DitherPicker was developed to aid icon designers by displaying a range of dithered patterns created from two solids you choose from the recommended palette of 34 colors. By using these dithered patterns, the perceived number of colors that is available to icon designers is immensely expanded.
One of the most difficult issues in computerbased publishing is color accuracy. The color models discussed in the previous section, RGB and HSB, are directly tied to the display device. These models are strongly devicedependent.
Many color models are inherently device dependent. Output devices such as display screens have different responses to the same color levels. Go into any store with a wall full of televisions for sale, all tuned to the same station, and you'll observe the differences. This color dependency is what makes WYSIWYG color control so difficult. A number of efforts at accurate, deviceindependent color display and control are impressive. These include PostScript Level 2 and Kodak's PhotoYCC. However, the quest for accurate color display and printing is not yet over.
6 . 3 . 3 Printing Color Models
Several color models have evolved for use in printing. Printing in color is the act of putting inks on paper. This action reverses the additive nature of color produced with light into a subtractive process. A red, green, and blue light source will appear white when combined. Red, green, and blue inks on paper will create a black area.
CMYK
The printing industry and many color computer printers use the CMYK (Cyan Magenta Yellow blacK) color model to create fullcolor printed images. This model is also know as process color. Four inks with the colors cyan, magenta, yellow, and black are combined using halftoning techniques to create the illusion of continuoustone color images. (See Section 6 . 2 Dots and Pictures in Chapter 6 Media and Document Integration for a discussion of halftoning.) The perceived subtle nature of color created by the small dots of CMYK colors are a side effect of our perception. Take a look at a color magazine photo with a magnifying glass. The color of a person's skin is not really there! The color mixing is in our brains.
SPOT COLOR
Spot color printing refers to the use of one, two, three, or more single distinct colors within the document. Common uses are to color all section headings a unique color or to color arrows that point out information.(8)
You must use a color system to specify a color and hope that what is actually printed represents what you specified. The most common system is the Pantone Color Matching system. Some of the more complete electronic publishing systems provide software support for the use of particular spot color systems.
WYSIWIP, or What You Select Is What Is Printed, can become problematic in the case of spot color. Color matching systems, as these things are known, exist as swatch books from which you select a color. These expensive books contain all the colors for a particular color system; for example, PANTONE or COLORCURVE. Note that the names of the various spot color systems are not generic color terms; instead, they are registered trademarks of their respective holders. The inks used to print these swatch books are specially mixed solid colors. If a print job uses a single spot color and the printer indeed uses the special ink, everything should be fine. If, however, the spot color will be produced using process (CMYK) colors, you may be in for a surprise. The use of CMYK process colors will be only a simulation of the selected swatch color. The Trumatch Swatching System is an exception to this rule and is based on CMYK process color. Its swatch book is indeed printed with process colors.(9)
Cost is another significant issue with color printing. For example, if you intend to produce color PostScript files, you typically must give the printer one file for each spot color. This can quickly raise your production costs. However, spot color is less expensive than fullprocess color printing. In fact, spot color printing is a costeffective means of using color. Typically, "you can expect to pay 25-35 percent above the cost of a one-color page for each additional press color."(10) However, only your printer knows for sure.
PANTONE SYSTEM
The Pantone Matching System is a widely used color specification system. The actual colors are a set of more than 533 colors, each with a name or number. You can purchase sample books with the colors printed on coated or uncoated stock. The color of an ink appears more saturated on coated stock because the coating prevents the ink from getting into the fabric of the paper.
The Pantone system is widely used largely because of the variety of swatch books available. These include small books, books with tearout chips, twocolor comparisons, tint selectors, and process color simulations, all on coated or uncoated stock.
Although the Pantone Matching System originated for the printing industry, it is also widely used in the computerbased, graphic arts industry. Programs such as FrameMaker and Adobe Illustrator allow you to select Pantone colors. These can produce reasonable facsimiles of the printed output right on the display screen. However, as discussed in the earlier section on Color Models, the display screen will not be an accurate rendition of the printed color. You will need a proof print to be sure.
