Chapter 5. Pre-press

5.3 Colour

Wayne Collins

The second challenge with implementing WYSIWYG for electronic documents that are imaged on substrates is managing colour expectations. Chapter 4 discussed the challenges of colour management from the perspective of how we see colour, measure it, and manage output devices with ICC profiles. In this chapter, we will explore colour management from the perspective of how we recognize and manage the ICC profiles that are embedded in client documents. We will also explore the preflight issues in managing spot colours in documents. This will also lead us to a discussion of trapping for lithography and flexography.

To design with colour in computer graphics software, we must understand how the software generates the colour values. Page layout and illustration software usually have several systems for creating colour on a page. The colour settings or colour preferences attached to a file can change from one document to another or in the same document restored from one computer to another. If a designer uses the RGB colour model to specify the colours in a document, the colours on the monitor can change depending on the translations done to the colour settings. This is a major turning point for designers creating documents intended to stay in the electronic media. No one pays much attention to how a particular colour of red is rendered from one web browser to another. Designers pay more attention to how the colours interact relative to one another in web-page documents. It is only when we image a computer graphic on a substrate that we must pay attention to rendering the exact hue of red from one device to another. Coca-Cola has very exact specifications for the red used in its documents that tie into its brand recognition. So designers for documents intended for imaging on substrates must use colour models that are proven to render exactly the same results from one output device to another.

Pantone Colours

This is a lofty ideal that the graphic communications industry aspires to. There are systems in place that are proven to render very accurate results. There are challenges in understanding the systems, and one wrong step in the process and accuracy is destroyed. The process starts with how a designer chooses colours in a document. The most-used system for choosing accurate colours was created by the Pantone company. Pantone has developed a library of ink recipes that are published as swatch books. A designer can buy a printed book of a library of colours that matches an electronic library that can be imported into computer software programs. Designers compare their on-screen rendering of a colour to the printed sample swatch. If a designer is developing a corporate identification package with logos that use Pantone 123 and Pantone 456, the designer can be assured that the documents he or she creates will be imaged with inks that have similar spectral values to the swatch books used to choose the colour. I say similar, because the swatch books fade over time, and the substrates the books are printed on don’t usually match all the substrates a corporate logo is imaged on.

It is also important to realize that the Pantone library was created to mix pigments for spot colour inks rather than process colour inks. Spot colours are mixed independently and must each be applied to the substrate independently. Process inks are only the four primary colours: cyan, magenta, yellow, and black. Process inks are transparent and are intended to be combined by halftone screening different percentages on a substrate to render any colour in the Pantone library. Spot colour inks are more opaque and are intended to be applied to a substrate one at a time, through distinctly separate printing units. Since most colour photography is colour separated to render the photo in only the four primary process inks, most documents are created intending to convert the spot colours to process colours. They can be imaged with the photographs in the document. A designer must know how many colours the output device is capable of when deciding which colours will remain as spot colours and which will be converted to CMYK process colours. Most inkjet and electrophotographic devices are only capable of imaging with the four process colours. Some lithographic presses have extra printing units that can print spot colours, so six- and eight-colour presses commonly print the four process colours and two (or four) extra spot colours in one pass. It is not uncommon to have 10- and 12-colour flexographic presses that image no process colours but use 12 spot colours. This is because, historically, flexo plates cannot consistently reproduce very fine halftone dots reliably. This is changing with the development of high-definition plating technology, so we are seeing more photographic content produced well on flexographic presses. Flexography is primarily used in the packaging industry where spot colours are very closely tied to brand recognition in retail outlets. A designer must be very aware of the imaging technology used to reproduce a document when deciding which colours will remain spot colours.

This is where the next round of challenges begins when preflighting (assessing) documents for imaging to substrates. If design elements stay as spot colours, it is a simple process to maintain the spot colour on the output device and to image with the appropriate ink or toner. Some software will not maintain the spot colour in a document easily in some situations. Usually, the problem comes with applying gradients to spot colours. It is very easy to introduce a median colour value on a spot colour gradient that is simulated with a process colour value. The screen version displays a nice smooth gradient that looks like what the designer intended to create. When imaging on a substrate, the gradient will have to be broken down into individual colours: from the solid spot colour to a value of CMYK and back to spot colour. It is very hard to recognize this by viewing the document, or even a composite PDF file. Viewing separated PDF files, or using a ‘separations’ tool in Acrobat, will show the problem before it gets to a printing plate.

There are also colour problems associated with nested files generated in different software. For example, if we create a magazine page with a headline colour named “PMS 123,” add a logo created in Adobe Illustrator with type in a colour named “Pantone 123,” and insert a PDF ad created in Apple’s Pages layout with a border specifying “PANTONE 123,” then even though they are all the same, colour-separating software will generate three separate spot colour plates for that page. The spot colours have to be named exactly the same and come from the same library. Some modern workflows include aliasing rules that will match numbered PMS colours to try to alleviate the problem. Colour libraries can be a problem as well, especially if our software allows the library to convert the spot colour to a process colour. The same colour library in two different versions of Adobe’s Creative Suite software can generate a different process colour recipe for the same Pantone colour. This is not a problem if all the document elements are created within one software package and all spot colours are converted to process colours. The problem arises when a designer places a graphic file from other software on a page with the same colour elements. A logo created in an older version of Adobe Illustrator will use that colour library to look up process colour recipes that can be very different from the recipes in a recent colour library used in Adobe’s InDesign software. So all the Pantone orange colours in a document are supposed to look the same, but do not because the spot colour to process colour conversion has not been done with the same library. The problem becomes worse when we combine files from different software vendors, as designers often have to do when building a document. It is common these days to bring together graphics created in Microsoft Office and Apple software and generate a final document with Adobe InDesign. The best way to create consistent documents for reproduction is to specify a common CMYK colour value that will print reliably on the output device.

Pantone also publishes a swatch book that shows the difference between the swatches run as spot colour ink mixes, and the same swatch printed as halftone screen builds of process inks. This is a designer’s most valuable tool for specifying process ink recipes. It also illustrates that many Pantone colours cannot be simulated very well using halftone screen values of the four process inks. It is very apparent that very vibrant orange, purple, and green Pantone spot colours are not achievable with process inks. There are systems like Hexachrome for colour separations that use more than just CMYK inks to extend the gamut of the Pantone colours that can be reproduced. There are also more and more inkjet and electrophotographic engines that will use extra spot colours to extend the colour range of the device beyond CMYK. The businesses that employ those devices usually know they are unique in the marketplace and have developed marketing tools to help designers use those capabilities successfully.

Accuracy in Design

If we reflect back to the concept of WYSIWYG for a moment, we can use the Pantone selection process to illustrate the challenge very well. If we ask a designer to choose colours for a document based on computer screen displays, we know that the RGB or HSL values they can select will be far too vibrant for reproduction with any imaging engine. To set proper expectations for WYSIWYG, we ask the designer to calibrate a monitor and select the proper output profiles to tone down the screen view and set more realistic expectations. We also ask that a print designer use printed swatch books to select from a library of specified colours and assign realistic CMYK process colour values to her or his colour palette. If those steps are followed, there is a very reasonable chance that the process will achieve WYSIWYG. However, it can break down in a few places. The spot colour swatch books set expectations about colours that cannot be achieved with process inks. When a mixture of spot colours and process inks are used, it is difficult to display both on the same computer screen with reliable colour. Graphics files can originate in different software with different libraries using different process colour recipes for the same Pantone colours.
There are also many spot colour libraries to choose from, and designers don’t know when to use each library. We have described why the Pantone library is a North American standard, and some of its limitations. There are other design communities in the world that use spot colour libraries that are included as choices in graphic creation software tools. There are almost as many spot colours to choose from as there are free fonts files to download from the Internet. Spot colour classification has led to thousands of discrete colours being given unique names or numbers. There are several industry standards in the classification of spot colour systems. These include:

  • Pantone, the dominant spot colour printing system used in North America and Europe.
  • Toyo, a spot colour system common in Japan.
  • DIC colour system guide, another spot colour system common in Japan.
  • ANPA, a palette of 300 colours specified by the American Newspaper Publishers Association for spot colour usage in newspapers.
  • GCMI, a standard for colour used in package printing developed by the Glass Packaging Institute (formerly known as the Glass Container Manufacturers Institute, hence the abbreviation).
  • HKS is a colour system that contains 120 spot colours and 3,250 tones for coated and uncoated paper. HKS is an abbreviation of three German colour manufacturers: Hostmann-Steinberg Druckfarben, Kast + Ehinger Druckfarben, and H. Schmincke & Co.
  • RAL is a colour-matching system used in Europe. The RAL Classic system is mainly used for varnish and powder coating.

The guiding principle for using any of these spot colour systems is to check that the manufacturer of the reproduction is using that ink system. The Trumatch library is quickly gaining favour as a tool for colour selection. That library of spot colours has been developed to be exactly replicated with process colour halftone screening. There are no spot colours a designer can choose from that library that cannot be reproduced well with standard process inks. As more computer graphics are being produced on digital imaging devices that only use CMYK, this colour library is becoming the choice for cross-platform or multi-vendor media publications.