RGB versus CMYK

The RGB color model technology mixes (R)ed, (G)reen and (B)lue light to form any of the millions of colors displayed on a modern electronic video display. RGB deals with light. CMYK refers to the four inks used in some color printing: cyan, magenta, yellow, and key (black) and is a color model for printing with ink.

If you attempt to print with ink an image from an RGB source, the result will look differently than what you see on your screen. The print version will be much duller and lifeless. CMYK is a representation of the print result so that the image colors stay true from the source file to the final print.

RGB uses an additive color model whereas white is the "additive" combination of all primary colored lights, while black is the absence of light. Consider that with light, all colors combined result in white.

CMYK uses a subtractive color model whereas white is the natural color of the paper or other background, while black results from a full combination of colored inks. Consider that with pigments the combination of all colors is black.

RGB is a device-dependent color model: different devices detect or reproduce a given RGB value differently, since the color elements (such as phosphors or dyes) and their response to the individual R, G, and B levels vary from manufacturer to manufacturer, or even in the same device over time. Thus an RGB value does not define the same color across devices without some kind of color management.

Comparisons between RGB displays and CMYK prints can be difficult, since the color reproduction technologies and properties are very different. A CMYK printer uses light-absorbing cyan, magenta and yellow inks, whose colors are mixed using dithering, halftoning, or some other optical technique. Images displayed on a computer monitor may not completely match the look of items which are printed if opposite color modes are being combined in both mediums.

Converting an RGB image to CMYK alters the look of the image on a video display. There is no standard formula for the conversion. Conversions are generally done through color management systems, using color profiles that describe the spaces being converted. It is not an exact science. Colors can be adapted or reproduced images manually adjusted prior to sending to print. It is critical to understand that only a CMYK image should be supplied for print.

Finally, a common convention is to utilize 72 dpi when working with RGB images and to use 300 dpi when working with CMYK images. This is becamse a computer screen uses 72x72 pixels to show a 1x1 inch image (72 DPI), printers use resolutions starting at 300 pixels for each inch in order to show the image in the same size and quality.