The printing process is based on four colors of ink: Cyan, Magenta, Yellow and Black (CMYK). Each color is added separately as the paper passes through the press. This is why you may hear it referred to as a Four Color Process or Four Color Printing. Also why you may see your printer refer to a file that will be printed on each side as 4/4. Meaning, four inks on the front and four on the back. A job to be printed only on the front would be 4/0, etc.
When designing, you need to make sure you are using CMYK Color during four specific times: New Document Setup, Swatches and Color Palettes, Embedding Photos and Artwork, during PDF Export.
Each color in the printing process is produced by a special blend of the four inks: Cyan, Magenta, Yellow and Black. It is a subtractive process, white from the paper is subtracted as dots of ink are added to it.
*Make sure you design using the right Color Panel and Swatch Libraries. If you begin working with the wrong Color Panel, Color Guide, or Swatch Library, you may be looking at colors in the RGB color spectrum that can’t be reproduced by the CMYK color process.
Web based images have an RGB color mode. Monitors display color by using Red, Green and Blue light. It is an additive process, when all three lights are combined you get white light. RGB can reproduce a wider range of colors than what can be reached using inks.
*The Web Color Palette is much larger than the palette for print. Many of these colors are unable to be reproduced by ink.