CMYK is an abbreviation of the colours CYAN (blue), Magenta (red), Yellow and Key (black). Since your packaging will be printed, CMYK is the right colour mode. If using pantones, or special colours for your design, these must be converted to the CMYK colour mode.

A special colour is a print colour used in addition to the basic colours. The term comprises both special colours for additional colour depth as well as effects. If you notice that one of your colours is not in CMYK mode and is therefore a special colour, you will need to change the corresponding colour setting for that colour type in the colour processing settings and change the colour mode to CMYK.