[JoinPrint info] - What is CMYK?
As a designer, it is essential to know when to use RGB vs CMYK – CMYK: cyan, magenta, yellow, and black (In the printing press days when plates were being used the black plate was typically call the “key” plate because it carried the important key information relating to the artistic detail.), and RGB: red, green, blue colors on projects. A good rule of thumb is anything dealing with the web should always be in RGB and printed material should be in CMYK. But very few designers and clients know why this is the standard.

Here’s why…

Back in the printing press days, to achieve color, each ink (cyan, magenta, yellow, and black) had its own plate. First the printer would lay down one color, wait for it to dry, lay down another color, wait for it to dry and so on. Printing presses still work on that same theory to this day with the exception that offset printers can use a “spot” color which can be added to achieve a specific color swatch (usually a Pantone color). As the printing age has progressed, the digital printer has come a long way, allowing to print in RGB as well. But the standard still stays the same – use CMYK on all printing needs, as the color will appear differently if printed in RGB.

Source: https://cruxcreative.com/rgb-vs-cmyk-when-to-use-which-and-why/