Why colors sometimes 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 gamut 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. In addition, different devices produce slightly different gamut 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 equality paper), and other natural variations, such as manufacturing difference in monitors or monitor age.
See Adobe Photoshop "help" for more information on:
Identifying out-of-gamut colors
Understanding color space and gamut
Why colors change
Calibrating and creating profiles
Creating profiles of your input devices
Creating profiles of your output devices
Calibrating and creating a profile of your monitor