CMYK printing is the standard in the industry. The reason printing uses CMYK comes down to an explanation of the colors themselves. CMY will cover most lighter color ranges quite easily, compared to using RGB. However, CMY by itself can’t create very deep dark colors like “true black,” so black (designated “K” for “key color”) is added. This gives CMY a much wider range of colors compared to just RGB.
The use of CMYK (cyan, magenta, yellow, and black) for printing has become kind of a trope for printers. But the reason why printing uses CMYK isn’t that well known, even to many graphic designers.
On the surface, it doesn’t seem to make sense. You might even have learned in elementary school that the primary colors — red, green, and blue (RGB) — are the primary colors, from which all the other colors come. After all, monitors, projectors, and television sets use red, green, and blue (RGB) to create all the other colors. Mixing some of these colors produces the secondary colors — cyan, magenta, and yellow. Mixing them all produces white.
Primary colors however, are arbitrary. There are several competing color models, and the theory behind color is quite complicated. However, partly for historical reasons, and mostly because most electronic screens are dark, we use RGB color models for most light producing imaging devices such as monitors and projectors. Combining red, green and blue light produces lighter colors, offering a good contrast to dark screens.
This all begs the question:
Why don’t we just use RGB inks in printers?
While there are a select number of rare printers that use RGB, there are a multitude of practical reasons CMYK printing will be the dominant model for a long time. To understand why printers generally don’t use RGB, we have to understand a few things. On a monitor or projector, combining RGB colored light creates lighter colors- great if you have a dark screen.
However, on print materials, colors are produced differently from how they are made on a computer monitor. Layering RGB inks on top of or close to each other produces darker colors because inks can only absorb and reflect different colors in the light spectrum, not emit them. RGB colors are already dark to begin with. This makes it difficult to produce lighter colors such as yellow or lime green as adding RGB colors will invariably result in much darker hues.
Why use a CMYK color model?
Using a CMY color model provides a workaround for this problem as cyan, magenta, and yellow are lighter than red, green, and blue. CMY will be able to cover most lighter color ranges quite easily, compared to using RGB. However, while CMY by itself will not be able to create very deep dark colors or a “true black,” so black (designated “K” for “key color”) is added to CMY so a much wider range of colors can be achieved compared to just RGB.
Is it important to convert my design file’s color model to CMYK?
Yes and no. Even without further intervention, most printers will automatically convert RGB data to CMYK on the fly, even without your knowing it. PrintPlace.com’s system likewise does this conversion automatically. In the vast majority of cases it’s not strictly necessary to convert your file yourself. InDesign Secret’s David Blatner explained the specifics of why exactly converting RGB to CMYK is usually unnecessary in the 21st century, at least when using Adobe InDesign.
However, we recommend starting your project in CMYK or at least converting from RGB before you send it in for printing for a few reasons. The apparent color change from RGB to CMYK can be dramatic in many cases, such as when your artwork has a lot of bright red in it. If you desire the best possible color accuracy on your final prints, you will want to consistently set the color models for your design files before submission. This may be critical in cases where brand colors have been specifically set to a color hex code and color swatch. In these case, yes you should definitely make sure you have set the correct color models.
The only time you wouldn’t need to convert to CMYK is if you’re ordering 1-color envelopes with Reflex Blue or Standard Red spot colors.
Source : Why Printing Uses CMYK