RGB vs. CMYK - Information for Typesetters & Designers
The range of color that can be perceived (e.g. by the human eye) or displayed (e.g. by a computer monitor or printing press) is a color gamut. Different devices have different color gamuts. That is, they can produce a range of colors that vary depending on the device. That is why a color image may look very different when viewed on different monitors, and why, although both Macintosh and Windows operating systems recognize millions of colors, only 216 of those colors are considered ‘safe’ for web use - they are the colors that can be consistently reproduced on both Mac and Windows-based computers. To make things even more complicated, colors are created and described differently depending on how they are displayed. Computer monitors, which display in RGB, are also described as using an additive color model (add colors together to create white), while printed materials, using CYMK to create color, can also be described as using a subtractive color model (subtract colors to get white).
RGB Color Space
RGB stands for Red, Green, Blue. Computer monitors, video cameras, television sets, etc. create color using the RGB model. Under this model, a device will add varying amounts of red, green and blue light to a black background. On a color monitor this translates into having three possible light sources for each pixel on screen. To get white, 100% of each color is transmitted. All other colors are created by varying the amount of red, green and blue displayed. RGB is considered an additive color model. When there are no colors, the result should be black.
CMYK Color Space
CMYK stands for Cyan, Magenta, Yellow, and Black. Four-color printed materials are created by combining various amounts of cyan, magenta and yellow inks. Theoretically, 100% of each of those colors should create a true black, but usually the result is a muddy, dark brown color, so black is added as a separate color. To get white (paper), no colors are added. CMYK is therefore considered a subtractive color model.
What's it Mean?
Because of the difference in how monitors and printed materials create color, there are often surprising differences between what is created on screen and what the final printed piece actually looks like. Some computer programs (such as Photoshop) have gamut warning capabilities, to let the user know if they have created a color on screen that can not be accurately reproduced on press. There are also several monitor and printer calibration hardware and software packages available. Currently the most accurate way to reproduce color is to refer to known benchmarks such as the Pantone Color System, which provides color swatches and formulas for generating accurate spot, process, hexachrome, etc. colors. Systems like this ensure both designer and printer have a common reference, regardless of how something looks on screen. When working with photographs or colors which must match exactly, get accurate proofs (Kodak approval, Cromalin, Matchprint, or similar) before the job goes on press.
Color for Electronic Distribution
(web sites, CD or online image galleries, etc.)
Note that different monitors will display color differently, depending on the technology, brand, age of the monitor, and whether or not it's been calibrated. Something that looks great on your flatscreen laptop may look completely different on the client's flatscreen, even if it’s the same brand as yours, and is almost certain to look different going from flatscreen to cathode-ray. Changes in greens and yellows seem to be the most noticeable - what was a beautiful gold on your screen is a sickly jaundice color on theirs. If designing for electronic media, either stick to the limited but known 'web-safe' palate, or view your design on multiple monitors to make sure there are no nasty surprises.