Why RGB colors are used in monitors while CMYK in printers?

RGB VS. CMYK: WHEN TO USE WHICH AND WHY

As a designer, it is essential to know when to use 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 when to use 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.
On the other end, computer monitors give off colored light known as RGB (CMYK is colored ink.). This is because monitors are limited on the color spectrum range that they can produce due to the cathode ray tubes. Computer monitors have a larger color gamut than printing that can be achieved, which is why a computer can display a million more colors than what can be achieved with printing. Printing deals with absorption and reflection of wavelengths of which we perceive as color (CMYK). Printing also has its own limited color gamut. A lot of times customers will note that something looked different on screen than it does on paper and it is because of the different limited color ranges that computer monitors and printing allows.
To go into further depth, RGB colors are also known as “additive color” because to begin with, there are no colors and the colors are being added together to achieve further colors or until the outcome is white (look at the color chart image directly below, the inside color is white because it is all the colors added together). This is because our eyes receive no reflected light and they perceive the color to be black. However, when you add portions of red + green + blue the outcome is the CMYK colors as shown below.

 While in return, subtract cyan – magenta – yellow – black and you will get the RGB colors. CMYK colors are subtractive for this very reason that it starts with all colors and when colors are subtracted the outcome is black (see below color swatch, the inside color is white). This is because the colors absorb the light.

 To further summarize what has been discussed, when it comes to deciding which color group to use, first figure out what the output will be. If the output will be on a computer monitor then RGB is the way to go. If the piece will be printed, CMYK is usually the standard and the best option