A freelance client once told me, after receiving five hundred copies of a brochure I’d designed for her (and for which she’d opted not to see print proofs), “I love it! It looks nothing like it does on my screen, but I love it!” While print proofs certainly help, producing a printed page that accurately matches what’s being displayed on-screen is a difficult task. The challenge arises from the fundamental difference in how the two media render color; in short, screens use light, and print uses pigment. RGB (Red, Green, Blue)
At this point, we’re all familiar with the RGB color model. It’s the way we see the world today—that is, it’s the way our laptops, monitors, smartphones, televisions, tablets, navigation systems, and even billboards display information. While standard resolutions have increased over the years, it’s still possible to see RGB in action. If you can get your hands on an old CRT (i.e., cathode-ray tube, i.e., pre-flat-screen-era) television, flick the old beast on and look closely at the picture (use a magnifying glass if you’re still in the antique shop). Close up, you’ll see that the screen is a giant matrix of tiny red, green, and blue lights that, at varying degrees of brightness, make up a cohesive, full-color image. Today’s high-resolution screens all function by the same principle.
RGB is known as an additive color model, meaning the more color you add, the more light you get. Thus, theoretically speaking, full Red + full Green + full Blue = pure white, or maximum light. Another way to see RGB in action is to attend a stage performance and look back at the spotlights facing the stage; it’s likely you’ll see separate red, green, and blue lights. Though it may seem counterintuitive, when these three colors shine in the same place, white light is produced.
CMYK (Cyan, Magenta, Yellow, and . . . blacK)
The CMYK color model, on the other hand, describes the way most printed material is displayed. While a whole slew of specialty ink colors exist, we’ll focus here on the four primary colors: Cyan, Magenta, Yellow, and Black. CYMK, in contrast to RGB, is a subtractive color model, meaning the more color you add, the less light you get. Thus, full Cyan + full Magenta + full Yellow = Black (i.e., the complete absence of light). This makes sense—keep adding paint to a page, and you’ll soon have black (or close to it). But why, then, is a separate black ink needed? Outside the theoretical realm (i.e., in the real world), due to unavoidable ink impurities, full C+M+Y can only give us a dull dark gray. A separate black ink is needed for punchy true blacks.
A separate black ink is also, of course, necessary for easy black-and-white printing; running separate (but identical) C, M, and Y plates would kill the economic advantages of monochrome printing. That said, black ink alone lacks dramatic punch. To compensate, designers and printers use “rich black,” which consists of 100 percent black ink plus a varying percentage of C, M, and Y.
RGB and CMYK may appear to be polar opposites, but they’re actually inextricably linked. In school, we learn that the primary colors are red, blue, and yellow, and that various combinations of each give us the secondary colors: purple, green, and orange. Not true! Printers’ ink is just like any other pigment-based medium (e.g. paint); the real primary colors are (you guessed it) Cyan, Magenta, and Yellow. So what happens when we combine these with one another? As we can see above, Magenta+Yellow gives us Red, Yellow+Cyan gives us Green, and Cyan+Magenta gives us Blue. Of course, it works the other way, too; shine a Red and a Green light in the same spot, and you’ll get Magenta. Try it with the other combinations and see what happens.
All that theory sounds nice, but . . .
Now that we’re all experts in color theory, let’s look quickly at why, in practice, things appear different on the page and on-screen. As a designer brings your book to life digitally (in RGB mode), light is emitted from a monitor directly to the designer’s (and subsequently your) eyes; for this reason, a design in RGB mode is vibrant and literally luminous. Before your book and all its component photographs, illustrations, etc., go to print, however, the files need to be converted to CMYK. While a good designer or pre-press operator can minimize the impact, an RGB-to-CMYK conversion typically results in slightly duller colors, as printed inks do not have the benefit of light shining through them. (Instead, light must fall upon the printed page and reflect back into the viewer’s eye . . . but I promised no more theory.)
Knowing how these two opposing but related color models function in theory and practice will help you understand the inevitable differences in how your book—or brochure, poster, billboard, whatever—looks on-screen and in your hands. Hopefully, with a good designer and a set of color proofs, your printed book will be all but indistinguishable from what you see on-screen.