Short answer
RGB is converted to CMYK through colour profiles: the RGB values are turned into device-independent colour (CIELAB or XYZ) using the source profile, then into the ink amounts that produce that colour on one particular press, paper and ink set using the output profile. Because every press condition needs different ink amounts, and because the output profile also fixes the ink limit and how much black replaces CMY, the same RGB colour has many valid CMYK values — the right one depends on where it will be printed.
The textbook formula — K = 1 − max(R, G, B), then C, M and Y from what is left — treats inks as ideal filters that each absorb exactly one third of the spectrum. Real inks are not: process cyan absorbs some green and blue, magenta absorbs some blue, and overprints do not add up neatly because ink sits on ink and on paper. The formula also ignores dot gain, the paper's own colour and how much ink the press can hold. It produces numbers, but they do not describe a colour, only a recipe with no kitchen specified. A real conversion needs measured data about the printing condition.
An output profile is built from a characterisation data set: a chart of roughly 1,500 to 1,600 CMYK patches printed under controlled conditions and measured in CIELAB. From that the profile maker builds a lookup table in both directions. Going from colour to ink (the direction used for RGB to CMYK) is the hard one, because many CMYK combinations produce nearly the same colour. The profile maker resolves that by choosing a black-generation strategy and a total ink limit, and by deciding how out-of-gamut colours are compressed for each rendering intent. Those choices are baked into the profile, so two profiles built from the same FOGRA data can still give different CMYK numbers.
Convert one mid-grey to coated and uncoated profiles and you get different CMY balances, because uncoated paper is bluer (brightened) and absorbs ink differently. Convert it with a light-black and a heavy-black profile for the same paper and you get different K. Convert it to a newsprint profile and the total ink drops sharply. All are correct for their condition. That is why a CMYK file is a statement about a specific press: CMYK values made for FOGRA51 printed on newsprint will over-ink and darken; newsprint values printed on coated stock look washed out.
Keep images in RGB with an embedded profile for as long as possible, and convert once, to the profile the printer names, ideally as late as the workflow allows. Choose the rendering intent per job: perceptual for photographs with many out-of-gamut colours, relative colorimetric with black-point compensation for most graphics and in-gamut work. Never convert CMYK to a different CMYK through RGB — that rebuilds the black and can turn pure black text into four-colour text that misregisters. Keep solid black type as 100 % K only, which colour-managed layout applications preserve if configured to.
Why: Text was converted from RGB black, which became a four-colour black that misregisters.
Fix: Set text to 100 % K only, or use a conversion that preserves pure black.
Why: It was separated for coated offset with total ink near 330 %, well beyond newsprint's limit.
Fix: Reconvert from the RGB original to the newspaper's profile, or use a device link that re-separates within its ink limit.
Why: Each step converts or reinterprets untagged data with a different default profile.
Fix: Embed profiles, keep colour settings consistent, and convert once at the end.
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
The characterisation data sets used to build standard offset profiles (such as FOGRA51 and FOGRA52) contain CIELAB values for the 1,617 CMYK patches of an ISO 12642-2 target printed under a defined printing condition.
Caveat: Older data sets such as FOGRA39 use a 1,485-patch target; the values are smoothed averages, not a single press sheet.
Source: FOGRA51 characterization data (premium coated offset, ISO 12647-2:2013); FOGRA52 characterization data (wood-free uncoated offset, ISO 12647-2:2013)
FactStrong evidence
More than one ICC profile can be generated from a single characterisation data set, differing for example in their level of black generation.
Source: CMYK Characterization Data (registry); ECI offset profiles (colour standards: offset)
FactModerate evidence
The ECI publishes separate ink-limited profiles (for example 300 % and 330 % variants) built from the same Fogra coated data.
ConventionModerate evidence
Converting once, as late as practical, from tagged RGB to the printer's named profile loses less than repeated or CMYK-to-CMYK conversions, which rebuild black and can compound gamut clipping.
Caveat: A convention of colour-managed workflows rather than a rule; some print workflows deliberately use device-link CMYK-to-CMYK conversions that preserve black.
Source: White Paper 40: Black-point compensation — theory and application; ECI offset profiles (colour standards: offset)
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.