A monitor emits light, paper reflects it. From that one physical difference comes everything that happens to colours between the design and the print.
RGB and CMYK are not two notations of the same thing but two different ways in which colour comes about. Below we explain why some colours from the screen cannot be reached in print, how large that difference tends to be and what to do so that the print does not surprise you.
RGB: colour from light
The RGB model rests on three colours of light: red, green and blue. The screen emits them in different proportions and the eye receives the sum. This is additive mixing, in which adding components brightens the image: all three at maximum give white, none of them gives black.
That is why a screen can show colours of a purity unreachable for ink. A deep blue, a vivid green or a neon orange are colours that exist as light and not as pigment.
CMYK: colour from ink
In printing, colour comes about the other way round. Paper reflects light and the ink absorbs part of it: cyan takes away red, magenta takes away green, yellow takes away blue. This is subtractive mixing, in which each further ink darkens the result.
The fourth component, black, is added for good reason. The three colour inks together give a dirty brown instead of a clean black, and at the same time lay down so much moisture on the paper that it cockles and dries more slowly.
Hence a practical rule: black text is printed in black alone, while large black areas use composite black, that is black reinforced with the remaining inks. The total coverage of all four should not exceed about 300% on coated paper.
Gamut, or what fits at all
The set of colours obtainable in a given system is called the gamut. The gamut of a screen and the gamut of print overlap to a large extent, but they do not coincide: each has areas the other cannot reach.
Most is lost on saturated colours. Practice shows that what goes astray above all are intense blues, clean greens and fluorescent oranges. Muted colours, skin tones, greys and most natural colours pass into print almost unchanged.
The good news is that in a typical photograph only a small part of the pixels lies outside the print gamut. That is why an average photograph looks similar after conversion, and the alarm is raised only by graphics with flat, heavily saturated areas.
How much does the colour actually differ
The difference between colours is measured by an index written as delta E. It is not a percentage nor a distance on a colour wheel, but a measure built to correspond to visual impression.
| Delta E | What the eye sees |
|---|---|
| below 1 | the difference cannot be perceived |
| 1–2 | perceptible to a trained eye in direct comparison |
| 2–4 | visible to anyone when the samples lie side by side |
| above 5 | a clearly different colour |
When well prepared material is printed on a calibrated machine we usually stay within limits the eye cannot perceive. The problem appears where a colour lies far outside the gamut and has to be replaced by the nearest one available.
How conversion works
Turning RGB into CMYK is not a calculation by formula but a mapping of one set of colours onto another, described by a colour profile. The profile holds the measured behaviour of a particular combination of inks and substrate, which is why the same file prepared for coated and for uncoated paper gives a different result.
The way colours outside the gamut are treated is chosen deliberately:
- Perceptual compresses the whole colour space, keeping the relations between colours. Colours lose a little saturation, but the transitions stay smooth. Good for photography.
- Relative colorimetric leaves colours inside the gamut unchanged and moves those outside it to the nearest one available. Better where the fidelity of particular colours matters, for example brand colours.
Conversion is done once, at the end of the work. Going back and forth repeatedly takes away saturation each time, because every conversion rounds values that can no longer be recovered.
Why the same photograph looks different on two monitors
Before we decide that the print is to blame, it is worth checking the reference point. An uncalibrated monitor shows colours in its own way, and factory settings tend to be exaggerated: boosted saturation and brightness look attractive in a shop but lie when you work with colour.
For assessing printed material a brightness of about 120 candelas per square metre and a neutral screen white are assumed, and the print is viewed in light of a known colour temperature. The same print under a bulb and under a fluorescent tube is two different colours, and that is not the printer's fault.
The most frequent surprises
- A neon green or blue from the screen that comes out muted in print. The colour simply lies outside the gamut of the inks.
- A black background in dark grey, because black alone was used on a large area instead of composite black.
- A blurred, soaked print where the total ink coverage is too high and all the inks were laid down close to maximum.
- Two different shades of the same logo on a leaflet and on a banner, because the materials were printed with different techniques on different substrates.
- A brand colour that misses the reference, despite a correct file. Some colours from palettes chosen on paper have no equivalent in four printing inks.
Check before you send it to print
The simplest thing is to see the difference for yourself. In our CMYK simulation you upload the artwork and see how it will behave once it moves to printing inks: which areas fall outside the gamut, how large the colour difference is and what happens when the substrate changes.
It is also worth checking the file itself: colour mode, bleeds, embedded fonts and image resolution. Our PDF file check does that, running through several dozen rules.
Summary
RGB is colour from light, CMYK is colour from ink. The first adds and brightens, the second subtracts and darkens. They are not competitors but tools for two different tasks: the screen and print.
In practice it comes down to three things: work in the space suited to the medium, convert once and at the end, and check critical colours in advance instead of hoping they will come out.
Do you have a project where the colour has to match? Take a look at our offer of printing. We will advise on the substrate and technique, and with brand colours we will check what is achievable before production starts.
