Two features that exist entirely for the printing press, that have no meaning on screen, and that cause a specific and expensive class of production error: spot colours and overprint. Both are invisible in an ordinary PDF viewer, both change what comes off the press, and both are routinely set by accident. This is what they are, how to see them, and how to avoid the classic failures.
What a spot colour is
Ordinary process printing builds every colour from four inks — cyan, magenta, yellow and black — laid down as overlapping dot patterns. A spot colour is a fifth (or sixth, or seventh) ink, mixed to a specific formula and printed from its own plate.
Reasons to use one:
- Colours process cannot hit. A vivid orange, a deep navy, most metallics and fluorescents live outside the CMYK gamut. A brand colour that looks slightly wrong in CMYK is a common driver.
- Exact consistency. A corporate blue that must be identical across every print run and every substrate is more reliably delivered by a mixed ink than by four-colour registration.
- Cost, on simple jobs. A two-colour letterhead printed with black plus one spot is cheaper than a four-colour process job.
- Special effects. Metallics, fluorescents, varnishes and spot UV are all handled as additional separations even though they are not colours in the ordinary sense.
In a PDF, a spot colour is a Separation colour space: a named colorant (PANTONE 286 C), a tint value, and an alternate colour space with a tint transform that says roughly what it looks like in CMYK or Lab. That alternate is what your screen shows you. It is an approximation and should never be trusted as a colour match — a Pantone chip under controlled lighting is the reference, not your monitor.
The spot colour failures
Unintended spots. A logo supplied as an EPS with a Pantone in it, placed in a document intended for four-colour process, silently adds a fifth plate. The printer either charges for it or converts it, and the first you hear is a surprise on the invoice. Check the separations before sending; see below.
Duplicate names. PANTONE 286 C, Pantone 286C, and PANTONE 286 CV are three different colorants as far as the file is concerned, and produce three plates of nominally the same ink. This happens when artwork comes from several sources. Normalise the names before output.
Spot used for something process should do. A spot colour where a process build would be fine adds a plate and cost for no benefit.
Converted when it should not have been. The reverse error: exporting to PDF/X-1a with "convert all spots to process" ticked turns your carefully specified brand ink into a CMYK approximation. Which is fine if intended and a problem if not.
Related background: CMYK vs RGB in PDF and colour management in PDF.
What overprint is
When two coloured objects overlap, the press has two options for the area underneath:
- Knockout (the default): the background ink is removed where the foreground object sits, so the foreground prints on bare paper. The colours do not mix.
- Overprint: the background ink stays, and the foreground prints on top of it. The inks mix physically.
Overprint has legitimate uses:
Trapping. A tiny overlap at the boundary between two colours, so that slight registration error on press does not reveal a white line. Modern workflows handle this automatically in the RIP, but manual traps still exist in older artwork.
Rich black over colour. Black text set to overprint on a coloured background avoids registration halos at the letter edges — one of the most useful applications.
Deliberate ink mixing. A varnish or metallic printing over process colour, or two spots creating a third colour where they overlap.
The overprint failures
These are the ones that produce reprints.
White set to overprint disappears. This is the classic. White in printing is the absence of ink — there is no white ink in process printing. Set a white object to overprint and the press prints nothing there, so the object vanishes entirely and whatever was behind it shows through. On screen the object is plainly visible. In print it is gone.
This one failure accounts for a remarkable share of print disasters, and it happens because overprint attributes are inherited, copied with objects, and set by default in some workflows.
Overprint on light colours over dark. A pale tint set to overprint on a dark background is invisible or muddy, because the pale ink cannot lighten what is beneath it.
Unintended colour shifts. A yellow object overprinting a cyan background prints green. Perfectly logical, entirely surprising if unintended.
Overprint preview off. All of these look correct in a normal viewer, because ordinary PDF viewers do not simulate overprint. The file looks right until it does not.
Seeing what is actually in the file
You cannot fix what you cannot see, and the default view hides all of it.
Acrobat Pro's Output Preview (Tools → Print Production → Output Preview) is the tool. It gives you:
- Separations list — every plate the file will produce, including every spot colour, with checkboxes to view each in isolation. This is where an unintended fifth plate becomes obvious.
- Simulate Overprinting — renders overprint the way the press will. Turn it on and the disappeared white object disappears on screen too.
- Total Area Coverage — highlights areas exceeding an ink limit you set, typically 300%.
- Object inspector — click any object to see its colour space, whether it is set to overprint, and its tint values.
Five minutes in Output Preview before sending a file is the highest-value habit in print production.
Acrobat's Preflight can also report and fix: profiles exist for "convert spot to process", "remove overprint from white objects", and similar. Fixups are safer than editing the artwork by hand.
Free options are thinner. Ghostscript can produce separations with the tiffsep device, which lets you inspect plates without Acrobat:
gs -sDEVICE=tiffsep -dNOPAUSE -dBATCH -sOutputFile=sep%d.tif input.pdf
Each separation is written as a file, so an unexpected fifth one is immediately visible. Background: ghostscript introduction.
gs -sDEVICE=inkcov reports ink coverage per page, useful for a quick ink-limit sanity check.
Setting them deliberately
In Illustrator and InDesign, overprint is set per object in the Attributes panel (Overprint Fill, Overprint Stroke), and both applications have an Overprint Preview mode (⌘/Ctrl+Alt+Shift+Y) that should simply be left on while working. Spot colours are swatches with the Spot colour type; the Swatches panel shows which is which by its icon.
In Acrobat, preflight fixups can add or remove overprint globally, but per-object editing is not practical — go back to the source file.
Standard practice worth adopting:
- Small black text: 100% K, set to overprint. Avoids registration fringing.
- Large black areas: rich black, knockout, not overprint.
- White objects: never overprint. Ever.
- Spot colours: named consistently, and audited before every send.
- Work with overprint preview on, so what you see resembles what prints.
If you are not printing commercially
Everything above applies to press work. If your PDF's destination is a screen, an office laser, or a digital press, spot colours are converted to process anyway and overprint is generally ignored or simulated. You still want to know if they are present — an unconverted spot colour in a file destined for a website is a sign the artwork came from a print workflow and may carry other print-oriented baggage, such as CMYK images that will look flat in a browser.
Summary
A spot colour is a named ink with its own plate; overprint decides whether overlapping inks mix or knock each other out. Neither is visible in a normal PDF viewer, which is why both get set by accident. Open Output Preview before every commercial print job, check the separations list for plates you did not intend, turn on Simulate Overprinting, and never let a white object overprint — it will not print at all.
