Setting your artwork to CMYK color mode
Ideally, you directly set your document to CMYK color mode when starting to work on your file. In case you have not done so yet, please follow this tutorial to change your file to CMYK color mode.
During our offset printing process, we use CMYK, which is a color model in which all colors are described as a mixture of the four process colors: Cyan, Magenta, Yellow, and Black.
To ensure that your document is in CMYK color mode, you can open Window → Document Info and verify that the file is in CMYK color mode.
If the document is not in CMYK color mode, please go to File → Document Color Mode → CMYK Color in order to change the color mode.