Short answer
Black ink alone (100 % K) is not very dark: on coated offset it reaches about L* 16, on uncoated about 33. Adding cyan, magenta and yellow under it — a rich black — makes it deeper, but too much colour turns it brown, green or muddy and exceeds the ink limit. Registration black (100 % of every plate) is for printer's marks only. A good rich black uses a modest, balanced CMY underlay chosen for the paper.
Process black is a single film of carbon-black ink at a thickness set by the press standard. It absorbs strongly but not completely, and some light scatters from its surface and from the paper around the ink film. On premium coated offset the black solid is aimed at about L* 16; on uncoated paper, where ink sinks into fibres and the surface scatters more light, it is about L* 33 — a dark grey. On newsprint it is lighter again. Large areas of plain K therefore look washed-out next to a photograph's shadows, which already contain C, M and Y as well as K.
Adding colour under the black absorbs more of the light that K lets through. In the FOGRA51 data, 100 % of all four inks reaches about L* 12.7 against 16.0 for K alone. The added colour also sets the black's hue: black ink on its own is close to neutral, but a heavy magenta or yellow underlay makes a warm, brownish black, and heavy cyan a cold one. A typical approach is a cyan-leaning underlay of a few tens of per cent of each colour, balanced so the result is neutral on that paper — a 'cool' or 'warm' rich black is then a deliberate choice, not an accident.
Three things push a black towards brown. Too much yellow and magenta in the underlay, which is common when RGB black is converted by a profile with light black generation and the CMY builds up to fill the shadows. Uncoated or yellowish paper, which adds its own warmth and lets the colour inks show through the weaker K. And over-inking: a 400 % 'registration black' fill far beyond the paper's limit does not dry properly, sets off onto the next sheet, and looks mottled rather than deep. On newsprint the brown cast of the paper itself is part of the look and cannot be printed away.
Body text and fine lines: 100 % K only. Rich black on small type blurs because four plates never register perfectly. Large solid backgrounds and big display type: rich black within the ink limit, with a trap or 'keep-away' so the underlay does not peek out beside white type. Photographs: whatever the profile's black generation produces. Registration black (all plates at 100 %) is a separate swatch meant only for crop and registration marks outside the trim, where it appears on every plate by design; using it for artwork is the classic cause of smeared, over-inked blacks.
Why: The underlay is heavy in magenta and yellow, often from converting RGB black.
Fix: Specify a balanced rich black (cyan-leaning underlay, total within the ink limit) as a swatch.
Why: Registration black or another near-400 % build far exceeds the paper's ink limit.
Fix: Replace with a rich black at or below the printing condition's tone value sum.
Why: Type set in rich black; plates misregister slightly.
Fix: Set text in 100 % K only and let it overprint.
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
FactStrong evidence
In the published characterisation data, the black solid alone is L* 16.0 on premium coated offset (FOGRA51) and L* 32.7 on wood-free uncoated offset (FOGRA52).
Source: FOGRA51 characterization data (premium coated offset, ISO 12647-2:2013); FOGRA52 characterization data (wood-free uncoated offset, ISO 12647-2:2013)
FactStrong evidence
In FOGRA51, 100 % of cyan, magenta, yellow and black together is L* 12.7, darker than 100 % black alone at L* 16.0.
Caveat: 400 % coverage is included in the characterisation chart but exceeds the tone value sum a coated sheet-fed press should run.
Source: FOGRA51 characterization data (premium coated offset, ISO 12647-2:2013)
StandardStrong evidence
ISO 12647-2:2013 says the tone value sum on coated paper should be below 330 % and must not exceed 350 % for sheet-fed offset, which rules out a 400 % four-colour black.
Source: ISO 12647-2:2013 Graphic technology — Process control … — Part 2: Offset lithographic processes
ConventionModerate evidence
Using 100 % K alone for body text and fine lines avoids the colour fringing caused by small misregistration between plates.
Caveat: A long-standing prepress convention; the standards define register tolerances rather than prescribing text builds.
Source: ISO 12647-3:2013 — quality standard for newspaper production; ISO 12647-2:2013 Graphic technology — Process control … — Part 2: Offset lithographic processes
Reviewed 29 September 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.