Colour management in UV printing: why colours look different
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Almost everyone experiences it with their first UV print: the design looks vivid on screen, but on the object it is darker, flatter or shifted in colour. This is not a defect but physics. The screen emits light itself, while the print can only reflect the light that falls on it.
Why the screen lies
A monitor mixes colours from light (RGB). It can display very bright, saturated colours that cannot be reached with ink. The printer works with CMYK, that is, with colour that absorbs light. The range of displayable colours, the colour space, is smaller in print.
On top of that, many screens are set too bright and too cool from the factory. What still looks fine there becomes too dark in print.
The substrate has a say too
UV ink is translucent, so it lets light through. Whatever lies underneath always shows through.
| Substrate | Effect without white | What helps |
|---|---|---|
| White, light | Colours close to the original | usually no white needed |
| Wood, cork | Colours become warmer and darker, grain shows through | White underprint if the colour needs to be right |
| Dark, black | Colours almost invisible | White underprint essential |
| Transparent (acrylic, glass) | Colours look pale, see-through | White layer as a cover |
| Metal, brushed | Metallic shine shows through, sometimes looks refined, sometimes busy | White fully or partly |
More on individual materials under Materials for UV printing.
White underprint: how much?
White is the foundation for colour on dark or transparent surfaces. Too little white lets the substrate show through. Too much white can make colours look a little paler, lengthens print time and increases layer thickness. Many machines allow several white layers or an adjustable density. How the layers interact is explained in the article Ink layers in UV printing.
ICC profiles: the translators
An ICC profile describes how a device reproduces colours. The printer software uses it to convert the colours in your file into ink amounts.
- Manufacturer profiles are usually built in on desktop machines. They are tuned to typical materials.
- Material-specific profiles exist for professional systems and RIP software. They take into account how the ink looks on a particular substrate.
- Custom profiles you create with a measuring device (spectrophotometer). This is mainly worthwhile if you print a lot on the same material and have to hit colours exactly.
Important: work through the chain cleanly once. File in the right colour space, software with a matching profile, no double conversion. How to set up the file is covered under Preparing print data.
Calibrating the monitor
Without a calibrated monitor you judge colours by feel. A calibration device measures the screen and creates a profile that corrects brightness and colour temperature. For print preparation the screen is usually set much darker than you are used to.
Even without a measuring device, a lot helps: turn the brightness down, switch off night mode and blue light filters, and judge colours in even light rather than in direct sunlight.
Test prints and colour charts
The most reliable route to good colours is a test print on the real material. Best of all is a colour chart: many small colour patches with known values.
- Create a colour chart with the colours you often need, such as company colours or typical skin tones
- Print it once on every material you use
- Keep the chart and label it (material, white setting, print mode)
- For new jobs, pick the matching colour straight from the chart
That way you see at once which value looks like what on wood, slate or acrylic, without guessing anew each time.
Skin tones
Skin tones are particularly tricky because the eye notices the smallest deviations. A slight green or magenta cast makes a portrait look unnatural at once. On wood, the warm natural tone of the material is added.
- Lighten photos slightly before printing, as the print almost always looks darker
- Print enough white under faces if the substrate is not white
- For portrait jobs, print a test tile with the face crop
Black
Pure black from the K ink alone sometimes looks more like dark grey in print. A deeper black is created when a little cyan, magenta or yellow is added. How much is usually handled by the software or the RIP. Too much colour on top of each other, however, increases layer thickness and, with hard ink, can worsen adhesion or flexibility. Small text, on the other hand, looks sharper in pure black because no colours can shift against each other.
Where the limits are
A CMYK printer simply cannot produce some colours:
- Fluorescent colours (neon): They glow thanks to fluorescent pigments. With CMYK they are only approximately possible.
- Metallic, gold, silver: Shine comes from reflection, not from colour. Without a special metallic ink, gold stays an ochre. One trick is to print on metal or mirror acrylic and deliberately leave out white so that the real shine shows through.
- Very saturated blues and greens from the RGB range often become duller in print.
For business use
Colour deviations are a common reason for complaints. Tell customers clearly in advance that colours look different on wood or metal than on a phone. For company colours, an approved test print before the production run is worthwhile. And keep your colour charts: when a customer reorders, you will hit the same colours again.
Continue with: Preparing print data and Ink layers in UV printing.