Colour Matching

As the leading UK digital label printer we recognise that matching colours on printed labels is an important and potentially complex topic and these labels print notes are intended to assist explanation and a shared understanding at a practical level. The notes address establishing printed label colour standards for new artwork including the use of press proofs and also the notes address matching existing printed colours when printing digital labels.

Colours are essentially mineral compounds from rock etc ground to microscopic particle size and suspended in a medium; the unit applied here is a dot of colour pigment suspended in medium and printed onto the self adhesive label stock. Light passes through the medium and pigment which is transparent and where certain wavelengths of light are absorbed or reflected and refracted. Absorption of some wavelengths from white light by pigment results in certain wavelengths emerging from the dot as visual colours so when we see red colour the material itself is not red but it reflects the red wave band from the white light full colour spectrum.

We need to be careful when assessing printed label colours that they are viewed under correct lighting conditions because if we view a printed label colour under say a fluorescent light of incomplete white light wave content then because not all white light wavelengths enter the dot a condition called metamerism can cause the viewed colour to be different to the colour perceived when viewed in white light of adequate intensity. We may take a garment to the window to see the colour in daylight for example.

Whatever labels print process is applied colour matching of the printed result to a colour labels target or printed label standard is usual and needs to be managed by all involved in the chain of events.

We see a wide range or gamut of colours in total at different levels of illumination in the natural environment and a less wide but still substantial colour gamut when viewed on a TV screen or monitor. Printed label artwork is usually created on a computer and viewed on a monitor where the colour gamut in RGB is bright and backlit on the screen.

In four colour process printed labels the artwork file is converted to a labels print file and printed on self adhesive label material with specified finish. The resulting colour gamut printed by mixing CMYK dots in proportion to the file content is viewed under white light where the colour is reflected or refracted to the viewer. The printed label CMYK process colour gamut is smaller than the RGB gamut viewed on a monitor so it is important where colour matters in detail to see a printed proof of new designs wherever possible.

Our digital label print process enables us to print press proofs of your designs using our production presses onto production label stock and finish. In this way you can access a CMYK printed label that you can cut and apply and approve without buying a plate. We can print a large number of different label designs in a single digital label press proof. Approved press proofs then become our contract standard to match.

Standards for assessment and Measurement New Printed Label Artwork For new artwork the printed label press proof maybe the first time the designer has seen the printed version of a design. Labelsprint take care to apply standard colour densities and settings and to record these for each printed label so that future print will match a proof standard.

However as Label Printers and Label Suppliers we recognise that for all manufactured things tolerances must apply; a threaded bolt will have a manufacturing tolerance expressed as say plus or minus 0.1mm and printed label colours will exhibit slight variations. We will always attempt to achieve a PMS or match a target but there are many factors in the label printing chain of events and even under closely controlled conditions variations will occur and all involved with label suppliers need to recognise this reality.

With colour the topic of matching a previous press proof and verification and measurement is not simple. We are optically sensitive to quite slight changes and this appears to be different for various colours for example we appear to be more sensitive to slight changes in orange colour than in green. ISO and CIE (see link to Gamut article) have devised precise measurement methods relating in part to the minimum perceived colour difference between a standard target and print result.

Values defined as units of Delta Error are applied and correlated to colour measured in L* a* b* (LAB) co-ordinate values. To establish precise measurements can involve consensus on lighting levels and becomes particularly difficult or subjective when assessing process colour illustrations made from half-tones of mixed CMYK component colours.

In general terms a tolerance of 3 Delta E is considered to be just visually perceptible when comparing an area of flat printed colour with another under the same lighting conditions; if we fold a piece of flat printed colour (not continuous tone) over another for comparison there can be a change brought about by the fold but this comparison is very strict and visual difference can be perceived within even 3 Delta E.

These issues need to be recognised as existing - should it ever be necessary to take expert advice in colour measurement of variation. Calibrated equipment and laboratory conditions of standard lighting apply so it is to be avoided and common sense must prevail. Also it will have been necessary to include colour test blocks in printed trim data to provide consistent measuring references.

Conventional print using printing plates often applies special wet ink colours to large areas of flat colour and even these experience variation throughout a print run. When these areas are printed by mixing CMYK to get say green, orange or blue then and especially for shades of colour the process print may reveal some slight unevenness and realistically we say that taking the value from a press proof as nominal then within 6 Delta E needs to be acceptable for the process and conditions in which low cost fast labels are digitally printed.

Standards for assessment matching external samples and PMS targets Colour match to existing print samples, colour material or PMS targets using digital CMYK label printing can be an iterative process depending on the colour content of the artwork file supplied. We cannot add pigment or medium to strengthen, change or weaken ink when printing digital label colours and densities unlike conventional high volume presses using expensive printing plates to produce high label volumes where ink can be adjusted on press.

For a digital printer to adjust colour this has to be achieved through the artwork file, it can be done on the fly to a degree but it needs the print to be produced, assessed, file adjusted and re printed and in some cases it may not be possible to achieve a target either because the colour sits outside the CMYK gamut or the flexo printed sample has bent away from file content and needs to be re-made. When applying Pantone standards it is also useful to apply the CMYK process colour PMS targets.

 

We will try to achieve a match on press and sometimes in discussion a press proof is necessary and there are creative ways to capture in a single press proof a number of optional shades.

Digital labels print offers the solution for small run labels and wide variety labels with advantages in speed and cost. Labels printed CMYK can exhibit some unevenness where flat areas of solid or half tone colour are printed. Our experience over 40,000 designs proves that in practice this is quite acceptable and the advantages well outweigh any slight variation.

Many printed label designers are engaging with digital label print to offer their customers the advantages and in this process they design for digital labels in variety, colour, content etc - as outlined above.

In practical terms the issue of colour matching is rarely discussed but could be an issue particularly matching a previous printed colour sample that may have shifted from the design file content over time. An important perspective is one of recognising the process involved and the need for some tolerance, particularly where half tone CMYK illustrations and continuous tone work maybe concerned. However in our experience as leading UK label printers and label suppliers with over 40,000 digitally printed label designs undertaken any difference is slight and acceptable when the process and options are considered. Press proofing can set a mid-point about which typically a realistic colour range tolerance needs to be acceptable this in LAB terms could be expressed as 6 Delta E in colour variation.

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