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Why colors don't match -- No device in a publishing system is capable of
reproducing the full range of colors viewable to the human eye. Each device operates within a specific color space,
which can produce a certain range, or gamut, of colors.
The RGB (red, green, blue) and CMYK (cyan, magenta, yellow, black) color modes represent two main categories of
color spaces. The gamuts of the RGB and CMYK spaces are very different; while the RGB gamut is generally larger (that
is, capable of representing more colors) than CMYK, some CMYK colors still fall outside the RGB gamut. (See Color gamuts
(Photoshop) for an illustration.) In addition, different devices produce slightly different gamuts within the same color
mode. For example, a variety of RGB spaces can exist among scanners and monitors, and a variety of CMYK spaces can exist
among printing presses.
Because of these varying color spaces, colors can shift in appearance as you transfer documents between different
devices. Color variations can result from different image sources (scanners and software produce art using different
color spaces), differences in the way software applications define color, differences in print media (newsprint
paper reproduces a smaller gamut than magazine-quality paper), and other natural variations, such as manufacturing
differences in monitors or monitor age.
Color numbers
Each pixel in an image document has a set of color numbers that describe the pixel's location in a particular color mode--for
example, red, green, and blue values for the RGB mode. However, the actual appearance of the pixel may vary when output or
displayed on different devices, because each device has a particular way of translating the raw numbers into visual color.
(See Why colors sometimes don't match.) When you apply color and tonal adjustments or convert a document to a different color
space, you are changing the document's color numbers.
Color profiles
An ICC workflow uses color profiles to
determine how color numbers in a document
translate to actual color appearances. A
profile systematically describes how color
numbers map to a particular color space,
usually that of a device such as a scanner,
printer, or monitor. By associating, or
tagging, a document with a color profile,
you provide a definition of actual color
appearances in the document; changing the
associated profile changes the color
appearances. (For information on displaying
the current profile name in the status bar,
see Displaying file and image information.)
Documents without associated profiles are
known as untagged and contain only raw color
numbers. When working with untagged
documents, Photoshop uses the current
working space profile to display and edit
colors.
Viewing color around you.
View your documents in an environment that
provides a consistent light level and color
temperature. For example, the color
characteristics of sunlight change
throughout the day and alter the way colors
appear on your screen.
SOURCE: PHOTOSHOP HELP
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