ColorSync was Apple's color management API for the Mac OS. The original 1.0 version was developed as a Mac-only architecture; however, Apple later co-founded the International Color Consortium to develop a cross-platform profile format which became part of ColorSync 2.0.
In ColorSync, the reference for defining colors was CIE XYZ space. All image input and output devices (scanners, printers, displays) had to be calibrated by providing a profile which defined how their color information was to be interpreted relative to XYZ color space. This profile might be provided by the device manufacturer, but for better-quality results, it might be generated by performing actual measurements on the device with a colorimeter.
Thus, when an image was scanned on a scanner, the image file would include a copy of the scanner's profile to characterize the meaning of its color information. Then when the image was sent to an output device, a matching process would convert the color information at the time of rendering from the source profile (that attached to the image) to the destination profile (that attached to the output device) so that the resulting colors would print or display as closely as possible to the original image.
Human color perception is an incredibly complex and subtle process. Also, different devices have widely different color gamuts or ranges of color they can handle. To deal with these issues, ColorSync provided several different methods of doing color matching. For instance, perceptual matching tried to preserve as closely as possible the relative relationships between the colors, even if all the colors had to be systematically distorted in order to get them to fit within the gamut of the destination device. Because the human eye is more sensitive to color differences rather than to absolute colors, this method tended to produce the best-looking results, subjectively speaking, for many common uses, but there were other methods that could be better in some cases.