Two ways of describing the same colour

RGB describes emitted light — a screen starts black and adds red, green and blue. CMYK describes reflected light — paper starts white and cyan, magenta, yellow and black inks subtract from it. These are different physical processes, not two spellings of one value, and their colour ranges do not coincide.

The RGB gamut is larger, and the gap is worst exactly where designers reach for colour: vivid blues and purples, bright greens, and highly saturated reds. Pure #0000FF has no printable equivalent — the closest ink mix is visibly duller. That is not a limitation of any particular press; it is a limit of what ink on paper can reflect.

Reading a CMYK build

A build is written as four percentages, normally C, M, Y, K in that order. So C40 M30 Y30 K100 means 40% cyan, 30% magenta, 30% yellow and 100% black.

Adding those numbers gives the total area coverage (TAC) — here 200%. TAC is the number that matters most in practice, because presses have a limit:

Stock	Typical TAC limit	Why
Coated (gloss, silk)	300–330%	Coating seals the surface; less ink is absorbed
Uncoated	240–280%	Porous surface absorbs and spreads more ink
Newsprint	220–240%	Highly absorbent, fast presses, low-cost paper

Exceed the limit and the sheet arrives wet: drying is slow, ink transfers to the next sheet in the stack, and the dark areas look muddy rather than deep.

Black is not one ink

The K channel exists because building dark tones from cyan, magenta and yellow alone produces a weak, brownish black and consumes enormous amounts of ink. Two builds are worth knowing by heart:

• K100 only — for body text and small type. Using a four-colour black for small text is a genuine mistake: if the plates misregister by a fraction of a millimetre, the text picks up coloured fringes that make it look blurry.
• Rich black, typically around C40 M30 Y30 K100 — for large solid black areas, which look noticeably deeper than K100 alone. The support colours add density that black ink cannot reach on its own.

A frequent error is using a rich black for everything, including text, because it looks better on screen. Keep them separate.

Warnings worth acting on

• Out of gamut. If the RGB colour has no CMYK equivalent, the conversion clips it. Decide which colour matters more than the exact value, or pick a printable alternative.
• Total coverage over the limit. Reduce the build for the stock, not for the screen.
• Rich black on small text. Switch to K100.
• Colour-critical brand values. Convert with a relative colorimetric intent, which leaves in-gamut colours untouched, rather than perceptual, which shifts everything slightly to fit.
• Converting twice. RGB to CMYK to RGB to CMYK loses colour at every step. Keep one master RGB file and convert once, at the end, for the specific press.

Common questions

Why does my printed blue look purple?

Because the blue you chose cannot be made from those inks, and the conversion moved it to the nearest printable colour. Ask the printer for their ICC profile and soft-proof against it before committing, rather than discovering it on the printed sheet.

Is 100% of each ink the darkest black?

It is the wettest, not the darkest. 400% total coverage is far over every stock’s limit, and the result is a smeared, slow-drying sheet. A rich black around 200–240% TAC gives a deeper black with far less ink.

Should I convert to CMYK myself or let the printer do it?

Convert it yourself. You see the shift while you can still change the design, and you can pick the right profile for the stock. Letting the printer convert means the first time you see the real colours is on the proof.