Using Spot Colours for Great Pantone Matches

Use Postscript Spot Colour naming to easily achieve accurate Pantone and other repeating spot colours. By carefully naming colours within your Postscript document using Adobe Illustrator or similar, you can ensure the closest match to a Pantone colour, or a L*a*b* defined spot colour on any media. This article explains the hows and whys.

Some of this document is an edit of an article by Max Derhak of Onyx Graphics, entitled Printing with Spot Colours. That document goes into far greater depth regarding the history of Postscript Spot Colours, and their use relating to offset presses. Here we have limited the scope, edited and added to that document so as to be most relevant to artwork production for wide format digital process colour. The original Max Derhak document is available for download 

Process Colour Printing

The Process method of getting colour on a press involves printing multiple inks at the same location. By “processing” the image to vary the amount of each ink at the same location a wider range of colours can be achieved than by printing the inks separately. Printing with combinations of Cyan, Magenta, and Yellow inks makes use of the human visual system by attenuating the amount of red, green and blue light. Therefore, all possible hues with a wide variety of tints and shades can be reproduced using only three inks. By adding black ink to this mix of process colours, less ink can be used (saving money), and a better grayscale as well as a larger print gamut can be achieved. Therefore, with these four colors of ink (CMYK) full colour image processing is possible.  

However, not every colour can always be reproduced using Cyan, Magenta, Yellow, and Black. In the world of offset printing, and with some digital machines, it is possible to use CMYK+Spot Colours. When creating an image (using Postscript or PDF) for such a press you want to be able to use spot colours if they are available, but use an appropriate combination of CMYK inks if the press is not set up with your desired spot colour. Because of this, the Postscript and PDF specifications require that both the spot colour name as well as a suitable replacement in terms of device colour channels are provided whenever a named spot colour is used. 

One problem is that since the spot colour alternate is defined in terms of device colour channels, there can be great variability in the observed colour of the spot colour replacement as defined by a Postscript or PDF file. This is because the observed colour for each channel of a printing device can vary widely from device to device, or even from print media to print media. 

Spot Colour Printing in Hudson’s Onyx RIP

The Onyx RIP was designed to output to devices that print with standard four colour (CMYK) as well as four colour + spot colours. It therefore supports the direct printing of spot colours as spot channels in a printer as well as named spot colour replacement that uses colour management to perform the colour replacement. It is that last feature that is so valuable to you the artwork creator if you want to either use a named colour, such as Pantone colours, or you know the L*a*b* values of the colour you want us to print. If the colorimetric value for a spot colour is known, then colour management can be used to find device colourant values that can better reproduce that spot colour. 

How it works.

The RIP software, developed with Pantone, includes a list (ColourTable or Lookup Chart) of L*a*b values for Pantone Colours. If it finds a match between the postscript label in your artwork and a known colour in it's Colour Table, then it processes those lab values, regardless of what is going on with the rest of the file. This gives you accurate Pantone Colours, without any complicated and expensive print/look/print/look iterations. The great thing is, you are not restricted to just the Pantone colours, you can set up corporate colours etc in the same way, ensuring accurate colour across multiple media. 

How to do it 

So lets say the logo you want printing is made of Pantone 368c and Pantone 210c, and you're using Adobe Illustrator to create your artwork. You need to set your logo components to the correct Pantone Colours. Luckily Adobe CS comes with more colour libraries than you'll ever need built right in, so you browse to those and select the colours you want. First though you make sure your setup is correct by browsing to: swatches/spot colors, as below, and making sure Spot Colors are set to l*a*b    Now you're ready to use Postscript labels. So select Swatches from your Tools Bar, click the icon in the top right corner to bring up the menu, select "open swatch library", then "Color Books" - WOW!! Look at all those! Most of the time with Pantone colours the book you want is "PANTONE  solid coated". With the element you want selected click on the Pantone Colour you want it to be and voila. Rather than a CMYK or RGB space dependant colour, your colour, when you select it, is listed as (in this case) "PANTONE 210 C". You have applied a Postscript Label.  Now this next bit is essential. The system is dumb. It doesn't know what the letters are. So if you use lower case when it should be upper, or put spaces where they shouldn't be, then your label won't be recognised. Don't go changing the standard format! When you save that document, make sure you keep it in a format that supports spot colours. ie. keep it in Illustrator or Indesign, and when you output to PDF make sure spot colours are kept.  When we receive that .eps or .pdf the label will be spotted, and the recorded lab values of that Pantone colour will be processed - giving you great Pantone colours immediately. Note: There are many Pantone colours that cannot be printed with process colours. If you're after one of those unfortunate colours, our colour managed workflow, with Pantone Spot Colour table, gives you the most accurate match that can be acheived.    

Personal Spot Colours 

Let's say you regularly print for a client who has a particular colour they want. But it's a paint chip, or a previously printed item, or.... you get the picture. So you create the artwork, and you create your very own Spot Colour. This is how... The process consists of two parts. 1) You need to label all elements of your artwork that you want to be the special colour with a Postscript Label, and because it needs to be exact name, you need to tell us that label. Remember, The system is dumb. the labels we use and you use must match. At this point it doesn't matter what colour you actually use in your artwork. Pick a random colour (one that isn't used elsewhere in your image), and give it the label you've decided upon. In this image I've picked a green, then I click on the colour chip to open the color tool in the top right and them the right hand corner of that tool to open the menu. on the menu I select Create New Swatch, which gives me this menu below.   I now change these details. The Swatch Name is the name you've got to give to Hudsons for your spot colour. Let's call it "CLIENT SPOT COLOUR 1". I need the Color Type to be Spot Color, and the Color Mode to be L*a*b. So we end up with this swatch colour.   I hit OK and now in the Swatch menu we find a colour chip...  Wherever I want the client's special colour in the artwork, I set it to this as if it were one of the pantone colours in the previous section. It does not matter at all that the colour we've used is Lime green, it is only the label we care about. Wherever our RIP software sees that label it's going to print the L*a*b colour we associate with that same label in our Colour Table. Just to be clear, even though your artwork shows Lime Green, lets say we register the l*a*b values for a dark blue against the label "CLIENT SPOT COLOUR 1", then when we print those Lime Greens will all be replaced with that dark blue. Which brings us neatly to the 2nd Part of the process.   2) You've given us the label "CLIENT SPOT COLOUR 1". Now you need to tell us what the colour is. If you've got something to match to, such as a paint chip, or other solid item, we'll measure the colour with a spectrophotometer and get its lab values that way. Other methods involve a little bit more experimentation, but we'll end up with l*a*b match to that colour that you are happy with. We register those l*a*b values against your label. Now whenever you send us a pdf or eps file containing the label "CLIENT SPOT COLOUR 1", you'll get the same colour. 

This is one method you can use to get great colour from Hudson Display Services Ltd, and because it's effectively an automated process, and because you don't need proofs to check your Pantones, it's fast and cost free.

Tips and Tricks

As with all rules there are exceptions. There could be several reasons why a replaced spot colour in a Postscript/PDF file may not appear to match the corresponding colour found in a published swatch book. Some of these are as follows: 

The colorimetric value of the colour is outside the gamut reproducible by the printer. Since the output profile is used to find the device values used to print the colour, if the colour is outside the printers gamut, gamut mapping will be used to find an alternate colour that is inside the gamut. In other words, if the Pantone colour can't be reached on the Process Colour Printer, the nearest colour that can be reached will be used. We can supply a gamut report for your job giving an estimate of how well all of the system table and user table colours can be reproduced by the print mode. (A print mode is the combination of printer/ink and media used for your job,)   This chart compares the required colour with the output colour (based on the output profile). A measurement of 2 or more here indicates that there will be an apparent difference in the colour. The higher the number, the bigger the difference, the further out the print will be to the desired target. In order to get better results, a larger print gamut may be needed. In some cases, we might be able to tweak the output settings to hit a particular colour. But sometimes a different media or print mode may be required to get better results. In other words, sometimes your colour just can't be hit on a particular media with a particular set of inks, and switching to a different media or different printer might get a better result.

There are some Pantone colours that simply won't be acheived.   We've talked about Pantone colours but for each of the standardized color matching systems (Pantone, HKS, RAL) ONYX Graphics (who make the software) has received from each vendor the officially recommended colorimetric values for each of the colour names in their respective colour matching systems. All work in exactly the same way. It is quite possible that these values can differ from the actual colour measurement from any particular swatch for a number of reasons. One is that since the production of swatch books is a physical process, there are tolerances and variability that can occur in the reproduction of each colour patch. The second is that once a swatch book is printed, the colour can change over time. (It is for this reason that Pantone recommends that swatch books be replaced on a regular basis). 

The reproduced colour could be a metamer to the required colour. Two physical colours are metamers if they appear the same under one viewing condition, but appear different under a different viewing condition. When this happens this is known as metamerism. The viewing condition involves both the viewer as well as the light source. Differences in either light sources and/or viewers can result in whether two metameric colors appear the same. In other words, you might put the printed colour next to the target colour in daylight and they match closely, but when you move them both into your office they differ - they haven't both changed in the same way as the lighting condition changed, so a difference becomes apparent.