What happens to a colour the press cannot print

Gamut is a solid, not a triangle

The familiar horseshoe diagram with triangles drawn on it is a slice. A real gamut is a three-dimensional solid in Lab, and its shape differs by lightness: a press can produce a vivid mid-tone orange it cannot produce as a light tint or a dark shade.

Where CMYK is smaller than sRGB, reliably:

• Saturated blues and violets. RGB blue (0,0,255) has no CMYK equivalent at all. It comes back as a dull purple-navy, and this is the single most common colour disappointment in print.
• Saturated greens, particularly bright emerald and spring green.
• Vivid orange and warm red at high saturation.
• Anything fluorescent. No process ink is fluorescent.

Where CMYK is larger than sRGB, which surprises people: some deep cyans and some rich yellows on coated stock exceed what an sRGB screen can show.

The four rendering intents

When a colour will not fit, the conversion must decide what to do with it. That decision is the rendering intent, chosen in the convert dialog.

• Perceptual. Compresses the whole gamut smoothly so relationships between colours are preserved. Out-of-gamut colours come in; in-gamut colours also shift slightly to make room. Correct for photographs, where a smooth gradation matters more than any one exact value.
• Relative colorimetric. Leaves in-gamut colours exactly where they are and clips out-of-gamut ones to the nearest reproducible colour. White is mapped to paper white. Correct for artwork, logos and anything with specified brand colours, which is most vector work.
• Absolute colorimetric. Like relative, but does not remap white: it simulates the source’s paper white, so a proof on bright white stock shows the yellowish cast of the newsprint you are simulating. Correct only for proofing one press on another.
• Saturation. Maximises vividness, ignores accuracy. Intended for business charts. Not for anything that matters.

Relative colorimetric with black point compensation is the right default for vector artwork. Perceptual is the right default for photographs. Most people use whatever is preset and never learn there was a choice.

Black point compensation

Without it, a source black that is darker than the destination’s darkest black gets clipped, and every shadow detail near it collapses into one flat tone. Black point compensation scales the whole tonal range so the source black maps onto the destination black, preserving separation in the shadows at the cost of a small overall lightening.

Leave it on for relative colorimetric. It is on by default in Adobe applications and in Scribus, and it is one of the few defaults that is correct.

Seeing the problem before you print

Soft proofing shows you the destination’s gamut on your screen. Free everywhere:

• Scribus: Preferences ▸ Colour Management ▸ Simulate printer on screen, plus the Out of gamut warning checkbox.
• GIMP: View ▸ Colour Management ▸ Soft-proofing, set the profile, and enable Out of gamut warning with a chosen marker colour.
• Krita: Ctrl+Y.
• Photopea: View ▸ Proof colours.
• Illustrator / Photoshop: View ▸ Proof Setup ▸ Custom, then Ctrl+Y; Photoshop adds View ▸ Gamut Warning.

A soft proof on an uncalibrated screen is a rough guide, not a match. It will still tell you truthfully which colours are unreachable, because that is a property of the profiles rather than of your monitor.

What to do when a colour will not fit

In order of preference:

1. Choose a different colour, from a printed CMYK chart, before the design is signed off. Cheapest fix by a wide margin.
2. Use a spot ink for that one colour. Costs a plate; reaches colours process cannot.
3. Accept the shift and tell the client what it will look like, showing a printed sample rather than a screen.
4. Change the substrate. A brighter, whiter, coated stock has a visibly larger gamut than uncoated. Sometimes the paper is the answer.

What not to do: push the CMYK values higher in the hope of more saturation. Beyond the press’s ink limit you get a wet, unstable sheet and no extra colour, because the ink is already absorbing everything it can absorb.

The one that catches designers out

RGB blue is the classic, and it is worth seeing rather than reading about. Fill a rectangle with #0000FF, soft-proof it against a coated press profile, and look at what comes back: a flat, slightly purple navy, several ΔE away from anything anyone would call blue. Now do the same with #00A0E9 (a cyan-leaning blue) and it survives almost untouched, because it sits near the cyan primary the press actually has.

That is the practical lesson of gamut in one experiment. Brand colours chosen near a process primary reproduce well everywhere. Brand colours chosen in the corners of the RGB cube reproduce nowhere, and will need a spot ink for the rest of the company’s life.