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.

What does it mean? White light is made up of equal quantities of red, green and blue light: our eyes interpret different mixtures of these primary lights as the sensation of colour. RGB is shorthand for the colour space used in editable Photoshop files.

Stands for: Red, Green, Blue

The three primary components of white light and the visible spectrum, RGB is a framework (or colour space) for analysing and measuring the hue of light sources. There are several ways of calibrating light sources for different purposes, but they all start by defining white light. From this starting point, each framework determines the red, green and blue components according to its structure.

Examples of alternative frameworks to Adobe RGB (default colour space in Photoshop): Hue, Saturation and Brightness (HSB)(eg Microsoft Word); Luminance, a,b (Lab) a very early (interwar) international colour space with a very handy use for lightening pictures in Photoshop.[cue future photoshop levels tutorial]

What does it mean? Trichromat is a collective name for cyan, magenta and yellow (CMY), the three transparent inks that are also known as subtractive primaries. In theory, at least, equal percentages of cyan, magenta and yellow should provide a neutral grey value. In practice the result is a distinctly muddy brown.

The lack of an ideal magenta dye has dogged the history of colour printing. This is just one reason why printers need to use four printing inks, using an adapted black ink to get a satisfactory colour output.

Using black ink to darken areas of an image (undercolour removal or UCR) also allows printers to save money on expensive transparent trichromat inks. It can help to manage certain practical constraints like the number of printing stations and plates that are available on a press to run a job.