Spot & process color

A process colour is printed from the cyan, magenta, yellow and black inks; a spot colour is an ink of its own, and a screen can only approximate it.

In short Model="Process" and Model="Spot" describe how a colour is printed, not how it looks. A process colour's appearance follows from its numbers and the document's colour profiles. A spot colour is a separate ink, so a viewer previews it from the colour description the swatch carries: its alternate, or its Lab values. Overprint, mixed inks and colour groups build on the same palette.

Process colour

A process colour is a recipe of the process inks (or, in RGB, of screen primaries). A CMYK process colour's ColorValue is the ink coverage itself: 0 100 100 0 is full magenta plus full yellow. To show it on screen, a viewer converts the four percentages through the document's CMYK profile. Without the profile, the same numbers can only be approximated.

Spot colour

A spot colour is printed with its own ink on its own plate: a PANTONE colour, a metallic, a varnish. The file cannot describe the ink physically; it carries two descriptions of how it looks:

• its own Space and ColorValue, which for library spots is a Lab measurement;
• its alternate, AlternateSpace and AlternateColorValue, the colour to use where the ink is not printed: on screen, in a composite proof, or when the spot is converted to process.

InDesign shows a spot colour on screen through its alternate. Library spots in InDesign's files have a Lab alternate equal to their Lab values; spots defined in CMYK have a CMYK alternate. A tint of a spot ink scales that preview toward the paper colour while keeping it on the spot's plate.

InDesign's Ink Manager can also prefer the spot's Lab values over its alternate ("use standard Lab values for spots"); that is an output and display setting, not part of the swatch.

In Paged: a spot colour with a CMYK alternate is previewed through that alternate, run through the document's CMYK profile, with any tint applied first. A spot with no CMYK alternate is shown from its Lab or RGB values. An Ink Manager setting displays Lab-defined spots from their Lab values instead; it changes the preview, not the plate the ink prints on. Supported · verified Tracked

Mixed inks

A mixed-ink swatch is a combination of a process colour and spot inks, or of several spot inks, at given percentages; a mixed-ink group generates a series of them. The format has room for them, but none of the 305 surveyed documents contains one.

In Paged: a colour with Model="MixedInk" is recognised and kept, but not resolved to a colour for display. Parsed, partly rendered

Overprint

By default an object knocks out what lies beneath it: the inks below are not printed where it is. Overprint keeps them, so the inks combine on the paper.

Attribute · Rectangle	Values	Paged	paged.set
OverprintFill	bool	read	frameOverprintFill

OverprintFill and OverprintStroke appear on page items and in object, paragraph and character styles; table cells have one per edge stroke (TopEdgeStrokeOverprint, …). The document's OverprintBlack preference (true in every surveyed file) makes 100 % black overprint on output whatever the object says. Overprint matters for print; on screen it only shows when a viewer simulates it.

In Paged: overprint is simulated per ink plate on the CPU rasteriser; elsewhere the preview approximates it in RGB. Supported · verified

Colour groups

A <ColorGroup> is a folder in the Swatches panel ("Brand colours"). In InDesign's files the groups are children of <Document> in designmap.xml. Each lists its members as <ColorGroupSwatch SwatchItemRef="…"> children, and each swatch points back to its entry in SwatchColorGroupReference. Groups organise the panel; they do not change any colour.

In Paged: colour groups are read and can be edited; rendering ignores them, as it should. Parsed, not rendered

Frequently asked questions

What is the difference between spot and process colour? A process colour is printed from the process inks; a spot colour is a separate ink on its own plate. In IDML the difference is the Model attribute on <Color>.

How is a spot colour shown on screen? Through its alternate, AlternateSpace and AlternateColorValue. For library spots InDesign writes a Lab alternate equal to the spot's Lab values.

Does a tint of a spot colour become a process colour? No. It stays the same ink at a lower percentage and prints on the spot's plate.

What do OverprintFill and OverprintStroke do? They let the fill or stroke print on top of the inks beneath instead of knocking them out. The effect appears in print, or on screen when a viewer simulates overprint.

Do colour groups affect rendering? No. A <ColorGroup> only organises the Swatches panel.