Why a print never quite matches the screen

Two ways of making colour

Anyone who has sent a picture to print and got back something duller than they saw on screen has run into a problem that is built into the physics. A screen and a sheet of printed paper make colour in opposite ways, and each can show colours the other cannot.

A screen makes colour by emitting light. Every pixel is a cluster of tiny red, green and blue lights, and the colour you see depends on how brightly each one shines. This is RGB, and it is called additive colour because the lights add together. Red and green light at full strength look yellow to the eye. All three at full strength look white, and with all three switched off the screen is black.

Print works the other way round. Ink sitting on paper absorbs some wavelengths of the light falling on it and reflects the rest back to your eye. This is subtractive colour, because each layer of ink removes part of the light. Cyan ink soaks up red, magenta soaks up green and yellow soaks up blue, and by overlapping them in different amounts a printer can steer which light gets back to the viewer.¹

Where the K comes from

Commercial printing adds a fourth ink, black, which gives CMYK. In theory cyan, magenta and yellow laid on top of each other should absorb everything and make black. Real inks are not that pure, and the result is a dark, muddy colour rather than a true black. Black ink also gives crisper text and fine lines, uses less ink overall, dries faster and costs less than stacking three colours to fake it.¹

The K is usually said to stand for "key". In colour printing the key plate is the one carrying the most detail, which the other plates are lined up against, and it is often the black plate.² You will often hear that K was chosen so that B would not be mistaken for blue. That explanation gets repeated a lot, but the key plate is the meaning that dictionaries of printing record.²

The idea of building a coloured picture from a few separate plates is old. Jacob Christoph Le Blon, working in London, was granted a royal privilege in 1719 for printing pictures from three mezzotint plates inked in blue, yellow and red, and he sometimes added a black plate so he could use thinner layers of colour.³ His 1725 book Coloritto is often described as a manual of colour printing, but the historian Ad Stijnman points out that it is really a guide to mixing paint for skin tones, with most of its colour added by hand.⁴ The screen side has its own landmark. In May 1861 James Clerk Maxwell showed a coloured image of a tartan ribbon by photographing it three times through red, green and blue filters and projecting the results on top of each other through the same filters, the same additive principle a modern display uses.⁵

The gap between them

Because one system emits light and the other filters it, each has its own gamut, meaning the range of colours it can physically produce.¹ The two ranges overlap a great deal without matching. The colours that suffer most going from screen to paper are the very saturated ones, pure reds, greens and blues in particular. Converted to CMYK they tend to look flat, and deep blues and purple-blues can print as dark blobs with little detail.⁶ Design software flags these as out of gamut and will swap in the nearest colour the press can manage.

The traffic is not all one way. A few printable colours, including a strong cyan and some very bright yellows, sit outside what a typical monitor can display accurately.⁷ So a screen preview of a print is an approximation too.

Printers have ways round the limits. Spot colours are inks mixed in advance to a set recipe, usually from the Pantone system, and they can reach pastels, fluorescents and metallics that four-colour process cannot.⁸ Another approach adds orange, green and violet inks to the usual four. X-Rite, which owns Pantone, says standard CMYK can match only about half of Pantone's spot colours, while the seven-ink version gets a good visual match for around 90 per cent.⁹

The practical lesson is that the mismatch is not a fault in the printer or the screen. It comes from the difference between light and ink, and the best results come from designing with the printed gamut in mind from the start rather than hoping a bright screen colour survives the trip onto paper.

---

1. Wikipedia, CMYK color model, Wikimedia Foundation.
2. Merriam-Webster, Key plate, Merriam-Webster Dictionary.
3. Jeremy Norman, Jacob Christophe Le Blon Invents the Three-Color Process of Color Printing, HistoryofInformation.com.
4. Ad Stijnman, Ink and light: Le Blon's Coloritto and Newton's Opticks, Thinking 3D, University of St Andrews, 2018.
5. MacTutor History of Mathematics, James Clerk Maxwell: The Great Unknown, chapter 5, University of St Andrews.
6. University of Ottawa, RGB and CMYK, University of Ottawa.
7. Santa Barbara City College, Color Modes, Santa Barbara City College.
8. RMIT University, Process (CMYK) and Spot (Pantone) colour printing, Colour Theory: Understanding and Working with Colour, RMIT University, 2023.
9. X-Rite, How the PANTONE EXTENDED GAMUT Guide helps Printers, X-Rite, 2016.