Printing 4-Color Ink on Tinted Paper
A PIE Blog and PIE Quick Tips reader wrote to me this morning asking about printing on colored paper stocks. He noted that the paper substrate changes the colors of the inks, and he asked what could be done to predict and control such a color shift. This is what I told him.
Since 4-color printing inks are transparent, the printing substrate will always affect the way the ink looks on the paper.
To be absolutely certain how a photo will look on a tinted stock, you really need to print a press proof (much as you would print a press proof for a duotone, since you can't absolutely simulate the colors in a duotone with the process inkset of a digital proofing device).
One way to control how the colors look, however, is to print an opaque white ink base beneath the 4-color image. This way the ambient light will go through the transparent process colors, reflect off a neutral substrate, and return to the viewer's eye. (The overall image will not be tinted by the background shade of the paper.)
However, keep in mind that (in terms of color theory and the effect of color on the human eye), the surrounding color of the paper (outside the immediate boundary of the 4-color image) will still affect the viewer's perception of the color within the image. (In other words, how a color looks will be affected by the colors around it.)
This is the best reason to say that only with a press proof can you know for certain just how an image will look. And, for consistency, this should be proofed in 5000 K light, which is sunlight (and which is the color temperature of the light used in the viewing booths at commercial print shops). Unfortunately, a press proof is expensive.
Another way to solve the problem is to mix opaque white or opaque silver ink with the process inks. This will reduce the effect of the substrate color on the overprinted process colors. However, silver ink may have a bit of a graying effect on the 4-color image.
When you consider these two options for making the base paper substrate less of an influence on the 4-color images, the main distinction between them is how the opaque ink obscures the paper substrate. In the first case, if you print a base layer of opaque white, you essentially block the paper and then print a color image on top of the white. This is similar to priming a dark canvas with gesso before starting an oil or acrylic painting.
The second option noted above, adding opaque white or silver to the 4-color images, just enhances the ability of the process-color images to block out the color of the underlying paper.
In the latter case, you can even adjust your approach slightly by printing the opaque silver ink in only a certain tonal range of the photos (the highlights, for instance). This could add a shimmering effect to specific parts of your photos, which would be quite dramatic.
Printing a Digital Proof
Another way to see how process color images will appear on colored stock is to print a digital proof on the tinted paper. This will not be an exact match, but it will give you a reasonable idea of what to expect. If you choose this route, it’s best to use a high-end digital press such as an HP Indigo, since the colors will be more accurate, and therefore your proof will look more like the final printed product.
Since most graphic designers have struggled with this question at some point, paper manufacturers fortunately have produced printed marketing collateral that directly addresses printing on colored stock. You can ask your print provider or paper merchant for a complimentary copy of these booklets, which often include various tints of paper such as cream, pink, tan, or light blue with printed 4-color images. You can get an idea of what to expect, and those who write these promotional books also usually include production notes listing tips and tricks they used to achieve the effects.
For instance, if you are printing Caucasian faces on a yellow-white paper, the faces may look jaundiced. Therefore, it is wise to tweak the tone curves for the various colors to compensate. That is, you may want to reduce the yellow in the facial tones (and also make whatever other tone curve changes your printer or the promotional books suggest).
Another way to gauge how your photos will look when printed on a tinted sheet (grey, tan, pale pink, or blue—or even just a yellow-white or cream stock) is to have your printer add your chosen paper stock at the tail end of another customer’s print job. This won’t account for the specific characteristics of your own 4-color halftones, but it will show you how a specific press sheet behaves when printed in 4-color process ink.
Once you have decided how to produce the 4-color images, there are a few options you should discuss with your printer.
The first is “wet-trapping" vs. “dry-trapping" the ink. For the former option you print the silver or white in-line with the cyan, magenta, yellow, and black. (You just add an additional ink unit on press, making this a 5-color rather than 4-color job.) This effect is more subtle; therefore, it may not achieve the results you want.
For “dry-trapping," you print the opaque inks first and then let them dry before overprinting the cyan, magenta, yellow, and black inks. This creates a more distinct separation between the opaque and transparent inks, allowing the 4-color photos to stand out more.
A similar approach is to have your printer use UV inks. These dry instantly when exposed to UV light. Unfortunately, not all printers have these inks (and, if they do, you may incur a surcharge for using UV inks rather than traditional offset inks). In this case, you could just have your printer dry-trap the print job. The only downside is that this option would take more time than using the UV inks.
What About Printing Light Text on Tinted or Dark Paper?
If your paper is truly dark, you may need multiple passes of an opaque ink for the text to effectively block the background paper (for example, printing multiple passes of a silver ink on a dark, navy blue press sheet).
This could be problematic; the ink coverage may not even be consistent. In such a case you may want to either hot-foil stamp the text with white foil (which would incur extra charges for the metal die used to cut the foil), or you may want to just use a white press sheet and reverse the type. (That is, the type would be white, or not printed. You would just print everything except the type.)
You could also use this technique to solve the problems that arise when printing 4-color images on tinted stock. If you printed the photos in 4-color process ink on the white of the press sheet and then print a light blue, pink, or cream background, there will be no cream or tan paper tint to affect the 4-color images.
[Steven Waxman is a printing consultant. He teaches corporations how to save money buying printing, brokers printing services, and teaches prepress techniques. Steven has been in the printing industry for thirty-three years working as a writer, editor, print buyer, photographer, graphic designer, art director, and production manager.]