Why your blue printed purple

Two completely different physical processes

A screen emits light. Start with black and add red, green and blue until you reach white. This is additive colour, and sRGB describes it.

Paper reflects light. Start with white paper and add inks that subtract wavelengths — cyan, magenta, yellow and black — until you approach a muddy dark. This is subtractive colour, and CMYK describes it.

They are not two notations for the same thing. They are two different sets of achievable colours, and neither contains the other.

The gamut problem, concretely

The colours sRGB can show and CMYK inks can print overlap substantially, but sRGB reaches much further into saturated blues, greens and oranges. The specific failures are predictable:

• Vivid blue — the classic. A saturated screen blue such as #0000FF has no CMYK equivalent, and the press produces the nearest it can, which reads as purple or as a flat navy. Every designer meets this once.
• Bright green — screen greens are far outside the four-colour gamut and print noticeably duller.
• Orange and vivid red — printable but weaker than they look on screen.
• Neon anything — not reproducible in four-colour process at all.

The colour did not change on the way to the press. It was never printable, and the screen had no way to tell you.

Soft proofing, free

Soft proofing simulates on screen what a given press and paper will produce. It is not exact — your monitor is still emitting light — but it will show you the gamut collapse before it costs you a print run.

• Scribus has full colour management with ICC profiles, a gamut warning, and proper PDF/X export. It is free and open source and it is the tool to use for anything going to a printer.
• GIMP: View > Colour Management > Soft-Proofing, with a CMYK profile loaded and Out of Gamut Warning enabled.
• Krita and Inkscape both support ICC profiles, with Inkscape's CMYK support historically the weakest of the set.

The paid industry tools are InDesign and Illustrator, named here because job advertisements name them. Scribus produces press-ready PDF/X files, and commercial printers accept them.

You need a CMYK profile to proof against. Your printer should tell you which one they use; common defaults are FOGRA39 for coated stock in Europe and US Web Coated SWOP in the United States. Free profiles are available from the ECI and Adobe.

Rich black, and the small text trap

A large area printed with 100% black ink alone looks washed out — a dark grey rather than a black — because a single ink layer cannot reach full density. Printers use a rich black: black plus percentages of the other inks, something like C40 M30 Y30 K100, which gives a genuinely deep black.

The critical exception: small text must be 100% K only. Four inks on a press are applied by four plates, and they never register perfectly. On a large area a fraction of a millimetre of misregistration is invisible; on 9-point text it produces coloured fringes around every letter and a blurred appearance.

So: rich black for large solids, pure K for text and thin rules. This single rule accounts for a large share of the difference between print work that looks professional and print work that does not.

Spot colours

Where a colour must be exact — a brand mark, a corporate identity — process printing is unreliable, because every press and paper shifts it slightly. A spot colour is a pre-mixed ink applied by its own plate, specified from a physical reference system, of which Pantone is the best known.

Two practical consequences. A spot colour can reach outside the CMYK gamut, so brands with a vivid identity colour often specify one. And each spot colour adds a plate, which adds cost — which is why two-colour printing, one spot plus black, is a real and economical option for stationery and simple work.

Pantone references are proprietary and the physical books are expensive. For most work you do not need one; for a brand identity that will be printed at scale, the studio or the printer will have one.

The rule that saves you

Never approve a colour from a screen for something that will be printed in quantity.

Look at a printed sample. That means a swatch book, a previous job printed on the same stock, or a proof from the actual printer. Paper changes everything — the same ink on uncoated stock is duller and spreads more than on coated — and no monitor simulates paper.

A digital proof from the printer costs a small amount and settles arguments permanently. For anything above a few hundred copies, it is the cheapest insurance available, and the one step that experienced people never skip.