CMYK is an abbreviation of Cyan, Magenta, Yellow, and blacK, a color model commonly used in printing. Most computer graphics, including charts and figures created with Excel, use the RGB (red, green, blue) color model. To convert a graphic from one model to another, use of a color management system is generally required. A color management system ensures that colors are conserved between different outputs (e.g., screen and print).
This is a companion discussion topic for the original entry at https://www.xltoolbox.net/colormanagement.html