in Production
CMYK, RGB and why the cover changes colour
A cover that glows on screen can arrive from the printer looking dull. The reason is physics, and it can be planned for.
It is one of the most common disappointments in book production. The cover looked vivid on screen, and the printed copy looks flatter, darker or simply different. Usually nothing has gone wrong with the printing. Screens and presses make colour in two different ways, and they cannot produce all the same colours.
Light and ink
A screen makes colour by emitting light. Each pixel mixes red, green and blue light, and all three at full strength make white. This is RGB, an additive model.
Printing makes colour by absorbing light. Cyan, magenta, yellow and black inks are laid on paper, and each ink subtracts part of the light that falls on it. The paper provides the white. This is CMYK, a subtractive model. The K stands for key, which in practice means black.
Because they work differently, the two models cover different ranges of colour, called gamuts. A typical screen can show many bright, saturated colours that a typical press cannot print. Vivid greens, electric blues, bright oranges and some purples are the usual casualties. When a file is converted to CMYK, those colours are moved to the nearest printable equivalent, which is usually duller.
Why the file matters
Covers for offset printing are usually supplied in CMYK. If a file arrives in RGB, the printer's system may convert it automatically, and an automatic conversion will not necessarily make the choices a designer would.
Converting deliberately, and early, is better. It lets the designer preview the result on screen, using the soft-proofing tools in design software, and adjust colours while there is still time. Designing in RGB until the last moment and converting at the end leaves the surprise for the proof.
Conversion depends on colour profiles, which describe how a particular device or printing condition reproduces colour. The printer can say which profile they use or recommend. Using the right one gives a more accurate preview and a better conversion.
Black is not simple
Black is a special case. For small text, a single black ink, 100 per cent K, is best. Text built from all four inks can show coloured fringes if the printing plates are slightly out of register.
For large areas of black, K alone can look greyish. A rich black, which adds some cyan, magenta or yellow beneath the black, looks deeper. But every printer sets a limit on total ink coverage, the sum of the four percentages, and exceeding it can cause drying problems and set-off. The printer's specification gives the limit and often a recommended rich-black mix.
Paper, finish and proofs
Paper shapes colour too. Coated papers hold ink on the surface and give brighter, sharper colour; uncoated papers absorb more ink and give a softer result. Lamination changes it again: gloss tends to deepen colours, matte to soften them.
A calibrated screen and a soft proof get close, but a screen still cannot show exactly how ink will look on a particular paper. The only reliable preview is a physical proof on the actual stock with the actual finish.
A colour that must be exact is best chosen from a printed swatch guide rather than from the screen. Then what was agreed is what gets printed.