The term CMYK refers to the four ink colors that make up the schema: cyan, magenta, yellow, and black. The three primary colors in addition to black to produce darker shades. To create an image, printers define four separate plates that make up a full image. One plate is used for each of the four colors (cyan, magenta, yellow, and black). This simple way to print, has worked effectively for hundreds of years.
PMS (or Pantone Matching System) colors are mixed before they are applied to a surface. The pure spot color, or PMS color, is a mixed ink that allows for a wider range of reproduced colors than the CMYK spectrum. Colors can be brighter and more vibrant with PMS colors. Each PMS color contains a unique hue, which can be matched by a number that is associated with the Pantone Matching System of swatches.
RGB stands for Red Green and Blue. These are colors of a monitor or screen. Images that are RGB are used for websites and social media. These should not be used in the in the printing process. Digital printers can convert RGB to CMYK but offset printing cannot.