What does it mean? Printer’s shorthand for colour printing, in which the software separates the colour content into four components (or channels in Photoshop).
Stands for: Cyan, Magenta, Yellow and Key (aka blacK)
These are the traditional set of transparent subractive primary colour inks used to print in colour on white paper. Referred to collectively as process colours in the context of four-colour printing, each one of the trichromats (CMY) absorbs portions of the visible spectrum, reflecting filtered light that our brains interpret as a sensation of colour.
Huh? Subtractive primaries?
The inks are transparent and each trichromat cuts out (subtracts) a predetermined range of wavelengths in the light that is reflected back from the white substrate. It may help to think of cyan as red-negative; magenta as green-negative and yellow as blue-negative, since this identifies the portions of the visible spectrum they block.
Industry standards. There are two that need to be identified:
SWOP (Specifications for Web Offset Publications) is a set of colour management standards used in the US and by US-owned print companies, eg Vistaprint
FOGRA (the specifications were once loosely referred to as “Euroscale”) defines colour management standards used in Europe.
Does this make a difference? Hell, YES! Picture files need to be converted from RGB light values to CMYK colour printing values. A standard RGB colour space (aka 24-bit colour) has over 16 million hues that need to be translated into a fraction of that number to be accommodated in the smaller range of colour combinations that are available in CMYK.
The factory default CMYK settings for a lot of image editing software is the US-based SWOP colour space. Converting an RGB original to a CMYK file should only be done with a clear idea of the colour management regime in place at the final printing destination. Converting from one CMYK colour space to another is just asking for trouble.
Papermaking standards on opposite sides of the Atlantic have been a factor in setting the agenda for colour management, as the brightness of white paper fixes the highlight end of the printable range. Another determining factor is defining the light sources under which colour printing is signed off.