Adobe PHOTOSHOP 5.0 User Manual page 72

64 CHAPTER 4
Choosing a Color Mode
RGB mode
Photoshop's RGB mode uses the RGB model,
assigning an intensity value to each pixel ranging
from 0 (black) to 255 (white) for each of the RGB
components in a color image. For example, a
bright red color might have an R value of 246, a
G value of 20, and a B value of 50. When the values
of all three components are equal, the result is a
shade of gray. When the value of all components is
255, the result is pure white; when the value is 0,
pure black.
RGB images use three colors to reproduce up to
16.7 million colors on-screen. RGB images are
three-channel images, so they contain 24 (8 x 3)
bits per pixel. RGB is the default mode for new
Photoshop images. Computer monitors always
display colors using the RGB model. This means
that when working in color modes other than
RGB, such as CMYK, Photoshop temporarily
converts data into RGB data for display on-screen.
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.
For a color illustration of subtractive color
in CMYK, see figure 4-3 on page 223.
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 CMYK Preview command to simulate the
effects of the change without actually changing
image data (see "Previewing CMYK colors" on
page 110). You can also use CMYK mode to work
directly with CMYK images scanned or imported
from high-end systems.