7 Things You Need to Know About 7-Color Printing

Content provided by Kodak.

7-color printing is one of the emerging standards in how printers are printing ECG, or Extended Color Gamut. Doing 7-color ECG can push some printers out of their normal comfort zones, but both the journey to and the destination of 7-color ECG can be very rewarding. Here are 7 things you need to know about 7-color printing:

The First Rule of 7-Color Printing Is: You're Printing with 7 Colors

It seems to go without saying, which is why I'm saying it: when you do 7-color printing, you're printing with 7 colors. It means picking a set of 7 colors which your ink vendor has designed to work all together as a process set, in terms of their lithographic properties, including opacity and trap, as opposed to just adding 3 more inks whose color names sound about right and throwing them on press behind the CMYK (or should that be KCMY?) inks you're running today. It means 3 more press units to take care of, and ensuring that all 7 are running well, because in 7-color printing, all 7 process colors matter most of the time.

SCTV: Second City Television, or Spot Color Tone Value?

Just to up the acronym quotient of this blog, let's talk measurement. SCTV, ISO 20654, is a new, standard method of calculating TVI (tone value increase) which has the benefit of working equally well for most inks, especially the non-CMYK portion of your 7-color ink set. Trying to use Murray-Davies-based EDA (effective dot area) will mean that you're going to end up with target EDA curves for the non-CMYK inks which look and behave significantly different than the CMYK targets. Using the SCTV calculation lets you use similar or even the same target for the non-CMYK inks, which makes managing the process of printing much easier.

The Three Faces of RGB

Let's face it: all digital images today start out as RGB of some sort, and we all tend to keep them that way in the master digital archive so that when it comes time to re-use those assets for other campaigns, we're starting out with something closer to the original artistic vision. RGB, though, does not represent color you can see. Only if you associate (consciously or by default) a particular type of RGB (common ones are sRGB and AdobeRGB) to the image will there be any meaning the triplets of RGB numbers you can get from your eyedropper tool in your favorite image editing suite. However, even the smallest of the common RGB color spaces (sRGB) has at least one corner (typically blue) poking out of and hence not reproducible with, the color space of most common CMYK ink sets.

The extra color gamut you get by adding the additional inks to make up a typical 7-color process ink set helps your printed work be more vibrant and saturated, better able to fulfill the creative intent of the customer footing the bill for the printing, versus using just CMYK inks.

CMYK: Four Shades of Grey

The industry is inexorably converging on a choice of either (or perhaps both) of KCMYOGV (orange-green-violet) or KCMYOGB (blue instead of violet) for 7-color printing. But note that everyone is still talking about keeping the CMYK portion of this. We don't want to totally abandon our CMYK 4-color process roots, because CMYK still has a great place in our color separations. CMY and K are still useful for making gray, which underlies most of the images we print.

In setting up a press for 7-color ECG printing, it's useful to be able to use a method like G7 to set up the CMYK portion of the 7-color process ink set. This allows for compatibility between the 7-color press you set up in this fashion and other CMYK presses. It's very common in the industry for some images to be supplied pre-separated to CMYK. While you could use color management to gamut-expand such images, this is generally not a very good idea. With CMYK as part of the 7-color process set and calibrated in similar fashion to other pure-CMYK presses, CMYK images can be printed as-is, while the RGB images and brand color graphic elements in the workflow can take advantage of the full 7-color ECG gamut for extra vibrancy.

Paper Is the Fifth Color

This is a phrase from a paper written by color scientist Trish Wales, to reflect the fact that what you print on makes as much of a difference to the color of the final result as what color inks get used to do the printing. This is equally true for 7-color printing systems as for CMYK print systems. Porosity, brightness, color, OBAs - you can have everything else the same, and have a completely different result. Just as with CMYK printing, you should expect that if you are using a printed guide book as your color reference, you need to ensure that either the paper you print on is similar to the guide book paper or else that you otherwise take the difference in paper into account when trying to hit the colors you see in the guide book. And, of course, if there is sufficient disparity between the papers, you'll need to temper your expectations as to color match for some colors - no point trying to reproduce bright saturated colors from annual brochure work on newsprint stock, of course.

Control, Don't Roll (the Six-Sided Dice)

Perhaps even more applicable for 7-color printing than for 4-color CMYK is the notion that the printing must be under control, that you should be printing using numbers as reference. Setup, then measure and confirm during printing should be the mantra.

Seven Can Be Your Lucky Number

Printing with 7 inks helps from both the top and bottom. From the bottom, leaving the same 7 inks in the press all the time reduces downtime due to wash-ups, enables economies of scale in printing using the same process inks all the time, and reduces makeready because the press printing units do not need to be warmed up with new ink for each job. This is the essence of printing Spotless - replace a family of brand color spot inks with a single ECG process set.

From the top, the additional capabilities of printing with 7 inks is a way to differentiate yourself from your competition. In addition, the process control work done in preparing for switching to printing with 7 colors in and of itself is a marketable quantity.