Illustrator: 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, and black) color models 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. In addition, different devices produce slightly different gamuts within the same color model. 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 brands of computer monitors, 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.