Adobe Photoshop CS5 One-on-One

RGB Versus CMYK

Whether an image is bound for inkjet or imagesetter, it must make

the transition from one color space to another. And that presents

a problem because monitors and printers use opposite methods of

conveying colors. Whereas images on a monitor are created by mix-

ing colored lights, print images are created by mixing inks or other

pigments. As illustrated in Figure 12-2, lights mix to form lighter

colors, inks mix to form darker colors. Moreover, the absence of

light is black, while the absence of ink is the paper color, usually

white. Just how much more different could these methods be?

PeaRl Of WISDOm

Given that monitors and printers are so different, it’s not surprising that they

begin with different sets of primary colors. Monitors rely on the additive

primaries red, green, and blue, which make up the familiar RGB color space.

Printers use the subtractive primaries cyan, magenta, and yellow, known

collectively as CMY. The two groups are theoretical complements, which

means that red, green, and blue mix to form cyan, magenta, and yellow—and

vice versa—as illustrated in Figure 12-3. The intersection of all three additive

primaries is white; the intersection of the subtractive primaries is a very

dark, sometimes brownish gray that is nearly but not quite black. Therefore,

a fourth primary ink, black, is added to create rich, lustrous shadows. Hence,

the acronym CMYK for cyan, magenta, yellow, and the “key” color black.

Fortunately, Adobe and most printer vendors are aware of the vast

disparities between RGB and CMYK. They are equally aware that

you expect your printed output to match your screen image. So

they’ve come up with a two-tiered solution:

• When printing an image directly to an inkjet printer or other
  personal device, leave the image in the RGB mode and allow
  the printer software to make the conversion to its native color
  space automatically.


• When preparing an image for commercial color separation—in
  which the cyan, magenta, yellow, and black inks are each printed
  to separate plates—convert the image to the CMYK color space
  inside Photoshop. Then save it to disk with a different filename
  to protect the original RGB image.
  I provide detailed discussions of both approaches in the following
  exercises. And I’ll provide insights for getting great looking prints
  with accurate colors along the way.

Lights intersect to make lighter colors

Inks overprint to make darker colors

Figure 12-2.

422 Lesson 12: Print and Web Output