Primary Colors Are Not Red, Blue, and Yellow

by: Thomas A. Whiteman

Copyright 2001, Graphic Arts Technical Foundation
Adapted from GATF SecondSight 53, a reprint of an article appearing in GATFWorld magazine.

Today’s imaging and publishing technologies have created more opportunities for color reproduction than ever before. Desktop publishing systems have simplified the technical side, allowing users, whether experienced or not, to make color separations, seemingly with the touch of a button, and color monitors can now reproduce about 16 million different colors.

Using color monitors and electronic prepress technologies doesn’t mean we can print 16 million colors, however. Today’s technologies have certainly extended the gamut of printable colors (between 5,000 and 6,000), but this gamut is still determined by ink and paper combinations, not by prepress equipment.

Even with all of today’s technologies, it is still necessary to understand color on its primary level—and to know that the primary colors are not red, blue, and yellow.

"Seeing" Color
Although extensive research has been conducted, we still do not completely understand what happens in the brain when we "see" color. The visual sensation known as color occurs when light excites two types of photoreceptors in the eyes—rods and cones.

Rods are sensitive to light. When light is low, rods can detect subtle differences between light and dark, but cannot detect color. As the light increases, the cones become sensitized and can detect the individual components of light which create the sensation of color. As the level of light further increases, colors can be perceived because color is inherent in light. Without light, there is no color.

To begin understanding color, we need to examine the components of what we perceive as "white" light, which actually contains the wavelengths for all colors. Passing "white" light through a glass prism bends the light beam. The wavelengths that make up the light bend at different angles and so we see them as different colors.

Light can be described as the wavelengths of the electromagnetic spectrum that are visible to us. The term "visible light," however, can be somewhat misleading. While humans do not "see" infrared or ultraviolet wavelengths because the human eye is not equipped with the sensors to detect them, the vision of many mammals does extend into these ranges.

What we humans can see falls approximately in the center of the electromagnetic spectrum, where the wavelengths range from 400 nanometers to 700 nanometers. We have agreed to call the sensation we experience at the 400-nm end of the spectrum "blue," and the sensation we experience at the 700-nm end "red."

The visual sensation of color is as subjective as our other sensations—taste, touch, hearing, and smell. Each person experiences the sensation of color differently, and many variables—the light source, surrounding colors, the viewer’s mood and past experiences, and differences in visual ability—influence color perception. But even if we all did see color the same way, we would still interpret and describe it differently based on cultural, social, and other life experiences. When describing color, you can’t assume that your idea of "sky blue" is the same as someone else’s.

Nonetheless, color communication is critical in printing, especially when printers are trying to reproduce the colors their customers want to see. Customers can often have difficulty describing the color effects they do want, finding it much easier to describe what they don’t want. Many times, printers find that customers may not be able to clarify what they want until after the first proof. The first round of off-press color proofs is frequently rejected because a customer had no reference point for comparison. When there are first and second proofs to compare, both customer and printer can usually pinpoint the effect being sought.

People often use terms like flat, muddy, dirty, too warm, too cold, brighter, more snap, and jump-off-the-page to describe color corrections, but what does jump-off-the-page mean? Desktop publishing software packages are not equipped with a jump-off-the-page button.

Approximately 8–10% of the male population and 0.5% of the female population have difficulty with color discrimination. The most prevalent difficulty involves differentiating between red and green hues. Color discrimination problems are not uncommon, but finding someone who is completely "color blind," or who sees only in shades of gray, is unlikely.

Many printers screen potential employees for color aptitude. The most common test of ability to perceive subtle color differences is the Pseudo-Isochromatic Plates from American Optical Corporation, and it is used in many eye care offices. Information about color aptitude tests is available from: B. Buckley, ASTM Standards on Color and Appearance, 1916 Race Street, Philadelphia, PA 19103; phone 215/299-5599; fax 215/299-2630.

Testing members of the printing industry for color aptitude is an excellent idea, but the individual responsible for color approvals, the print buyer, may not have been tested and may not realize that his/her color communication problems could stem from a difficulty in perceiving color differences.

Reproducing Color
Despite all the ways we think we have to reproduce color, there are only two basic methods—additive and subtractive. The additive process starts with black—the absence of light—and involves transmitted light before it is reflected by a substrate. The subtractive process starts with light already present and reflected from an object. Both processes are trichromatic; that is, they are based on the theory of using three primary colors to create all other colors.

Understanding the principles of the two systems is the foundation for understanding the many facets of the color reproduction process in printing. It’s the basis for understanding tone reproduction, gray balance, and color correction—all crucial in achieving proper contrast, color balance, and color hue in halftone reproductions.

Additive Color Process (RGB)
As mentioned, the additive color process begins with black, or the absence of light and therefore no color. And it involves transmitted light before it is reflected by a substrate. Adding and mixing the three primary wavelengths of light (red, green, and blue) in different combinations produces a full spectrum of colors. Adding all the primary colors in relatively equal amounts produces "white" light. Computer monitors, television screens, projection TV, and stage lighting are based on additive color.

Three beams of light, each projected through either a red, green, or blue filter, can illustrate how additive color works. The beams are adjusted so they overlap.

The filters in this case do not work as we might immediately imagine. For example, the blue filter over the light source does not "filter out" the blue wavelength but, rather, blocks the red and green wavelengths, allowing the blue wavelength through. The green filter allows the transmission of only green light, blocking the blue and red wavelengths, and the red filter transmits only red light, blocking the green and blue wavelengths.

Adding and combining the blue and red components of light creates the sensation we call "magenta." Similarly, combining the blue and green wavelengths creates the sensation called "cyan," and combining the red and green wavelengths produces the visual sensation of "yellow." "White" appears where blue, green, and red overlap.

Mixing any two of the additive primary colors will always produce another color, called a "secondary color." The secondary colors in the additive process are cyan, magenta, and yellow—which are the primary colors of the subtractive process.

Subtractive Color Process (CMY)
The subtractive color process is based on light reflected from an object and which has passed through pigments or dyes that absorb or "subtract" certain wavelengths, allowing others to be reflected. The primary subtractive colors—cyan, magenta, and yellow—can be combined to form red, green, and blue as secondary colors. Combining the ideal subtractive primaries in equal amounts produces black.

The subtractive color process is what allows us to see color in the objects around us. A green ball, for example, appears green in white light because the colorants in the ball absorb the red and blue wavelengths and reflect the green. Of course, in a light source that is minus a green wavelength, the ball would appear black because there would be no green wavelength for the ball to reflect back.

It is at this point that many people start becoming confused, especially because we’ve just described the green ball—whose color is based on the subtractive process and reflected light—in terms belonging to the additive process (transmitted light and RGB wavelengths). Remember, whether we are talking about an additive color or subtractive color process, the primary colors of the light we see come from the red, green, and blue wavelengths of the electromagnetic spectrum. The primary colors of the additive and subtractive processes differ, but we still need to talk about RGB wavelengths.

Another source of confusion can be traced to the way most of us learned about "primary" colors—with a box of crayons in grade school, where we were taught that the "primary colors" are red, blue, and yellow. Many people in the printing industry, especially in the pressroom and in interactions with clients, perpetuate this error by calling cyan "blue" and by calling magenta "red."

Printing Color
Printing is based on the subtractive color process and is usually done on a white, or relatively white, substrate. And just like any other object, the substrate appears "white" in white light because it reflects all the wavelengths of light (RGB). We can say, then, that all colors are in the paper. To reproduce color on the paper, we use transparent pigments (the process inks cyan, magenta, and yellow) to filter out the RGB wavelengths in various combinations.

Considering the transparent process inks as our eyes perceive them (as RGB wavelengths) rather than the way they are combined on paper to reproduce, we find that they are created by combining 2/3 of the wavelengths of light and subtracting 1/3.

For instance, what we perceive as "cyan" reflects the blue and green wavelengths (2/3 of the total) and absorbs the 1/3 that is red. We perceive as "magenta" the process ink that reflects the blue and red wavelengths (2/3) and that absorbs the green (1/3). We perceive as "yellow" the process ink that reflects the red and green wavelengths (2/3) back to the eye while absorbing the blue (1/3).

In theory, combining all three process inks should prevent the reflection of all the wavelengths of light, creating black. Process inks—even the best ones—do not have ideal absorption and reflection characteristics, and when CMY inks are combined, some light is still reflected back to the viewer. The result is a brownish hue rather than black. In order to print an acceptable black, we need to use black ink in the color printing process.

Paper has a significant effect on color reproduction. Paper reflects unabsorbed light back to the viewer, and a more reflective surface like that of a coated paper can be used to produce a wider range of colors than an uncoated paper. The rough surface of an uncoated paper scatters the light, reducing the amount reflected back to the viewer.

Viewing Color
Since color is the reflection of light, it is easy to understand why the kind of light in which we view color can affect our perception of it. Even though one light may appear to be "white," it may contain slightly different wavelengths than another seemingly white light. We’ve all seen the effect of viewing a photo under incandescent light and then under fluorescent light.

The light under which we view color is a variable that can be controlled. Standardizing and controlling color viewing and proofing conditions can eliminate discrepancies in the color approval process and add at least some consistency to color perception and communication.

Standards for viewing reflection prints and transparencies in the graphic arts are contained in ANSI PH2.30-1989 and ISO 3664-1975. Although both standards are very similar, work is nearly complete that will update and enhance both, and bring them into even closer agreement. (The updates will not change the fundamental requirements.) These standards are an attempt to reduce as many variables as possible in the color evaluation process in order to improve color communication.

The easiest way to achieve the standard recommended viewing conditions is to use a commercially manufactured viewing that is designed to meet the requirements of the standard. The following requirements are basic:

• A light source that simulates CIE Illuminant D50. This light source emulates a phase of daylight illumination that has a relatively balanced output of RGB light. (Manufacturers recommend changing lamps about every 2,400 hours because the color temperature changes as lamps age.)
  
• An angular relationship between the light source, print, and observer that minimizes the amount of light specularly reflected toward the observer’s eyes on or near the normal to the center of the print.
  
• A warmup time as recommended by the manufacturer (usually 10–15 minutes) so that the lamps can reach a stable condition before color viewing takes place.
  
• Gray paint (approximately 60% reflectance, i.e., Munsell N8/) for the viewing booth walls to provide a consistent surround and prevent adjacent colors from affecting color temperature.

It is also helpful to keep the area inside and surrounding the viewing booth clean and uncluttered. Pictures, press sheets, posters, or other brightly colored material in or near the booth can affect color viewing.

With the advent of digital proofing, many color proofs that match under one lighting condition may not match under another. "Metamerism," as this effect is called, usually happens because the colorants available for use in inkjet, dye sublimation, and other proofing technologies do not have the same spectral reflectance as the pigments used in the inks they are intended to simulate. The color technologist can achieve an excellent match under a single (standard) lighting condition, but not across a wide range of conditions.

Conclusion
Color judgments of photographic originals, color proofs, and printed samples play such a large part in print production that understanding how we perceive and reproduce color is an obviously important aspect of the process. Even though color perception is as subjective—and sometimes as arbitrary—as an opinion, color knowledge can help the customer, the printer, the color separator, and the press operator find enough common ground to facilitate color communication and understanding. Knowing that the primary colors are not red, blue, and yellow is a good start.