Adobe PHOTOSHOP 6.0 Manual page 121

RGB images use three colors, or channels, to
reproduce up to 16.7 million colors on-screen;
the three channels translate to 24 (8 x 3) bits of
color information per pixel. (In 16-bit-per-
channel images, this translates to 48 bits per pixel,
with the ability to reproduce many more colors.)
In addition to being the default mode for new
Photoshop images, the RGB model is used by
computer monitors to display colors. This means
that when working in color modes other than
RGB, such as CMYK, Photoshop temporarily uses
RGB mode for display on-screen.
Although RGB is a standard color model, the exact
range of colors represented can vary, depending on
the application or display device. Photoshop's
RGB mode varies according to the working space
setting that you have specified in the Color
Settings dialog box. (See "About working spaces"
on page 125.)
CMYK model
The CMYK model is based on the light-absorbing
quality of ink printed on paper. As white light
strikes translucent inks, part of the spectrum is
absorbed and part is reflected back to your eyes.
In theory, pure cyan (C), magenta (M), and yellow
(Y) pigments should combine to absorb all color
and produce black. For this reason these colors are
called subtractive colors. Because all printing inks
contain some impurities, these three inks actually
produce a muddy brown and must be combined
with black (K) ink to produce a true black. (K is
used instead of B to avoid confusion with blue.)
Combining these inks to reproduce color is called
four-color process printing.
The subtractive (CMY) and additive (RGB) colors
are complementary colors. Each pair of subtractive
colors creates an additive color, and vice versa.
Subtractive colors (CMYK)
CMYK mode
In Photoshop's CMYK mode, each pixel is
assigned a percentage value for each of the process
inks. The lightest (highlight) colors are assigned
small percentages of process ink colors, the darker
(shadow) colors higher percentages. For example,
a bright red might contain 2% cyan, 93% magenta,
90% yellow, and 0% black. In CMYK images, pure
white is generated when all four components have
values of 0%.
Use the CMYK mode when preparing an image to
be printed using process colors. Converting an
RGB image into CMYK creates a color separation.
If you start with an RGB image, it's best to edit first
and then convert to CMYK. In RGB mode, you can
use the Proof Setup commands to simulate the
effects of a CMYK conversion without changing
the actual image data. (See "Soft-proofing colors"
on page 131.) You can also use CMYK mode to
work directly with CMYK images scanned or
imported from high-end systems.
ADOBE PHOTOSHOP 6.0 111
User Guide