CMYK stands for Cyan, Magenta, Yellow, and Key (black). The CMYK is a color model used in color printing. It is also commonly referred to as Four-Color or Process Color. The process of applying CMYK colors is usually implemented in the order of the acronym. The CMYK color model works by partially or entirely masking different colors, typically on a white background. This model is also called a subtractive method because the inks subtract the brightness from the white background. The CMYK color model is used for print material. It functions by using the ink to display different colors.
The other type of color model is called RGB. RGB stands for Red, Blue, and Green. As opposed to the CMYK model, the RGB color model is an additive model. It is classified that way due to the fact that the colors, (red, blue, and green) are added together in various ways to achieve a wide array of colors. The RGB color model is used for computer, or digital displays. Computers usually display RGB using 24-bit color. The colors in the RGB model are much brighter that the CMYK model.
At some point in the design process your images must be converted from RGB to CMYK in order to be able to print the images. However, not all colors are able to be transferred from RGB to CMYK. Those colors are said to be “out of the CMYK color gamut.” There are many different software programs that will convert RGB to CMYK. Some colors convert very well and some do not. Typically the brighter the color the harder it is to convert.
An example of that is:
An example of colors that are transferred at a better quality are: