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