Colour Reproduction

Does printing work like this too?

Printing uses subtractive colour synthesis, with four basic colours: cyan, magenta, yellow and black. Unlike the additive synthesis in our eyes and in computer screens, subtractive synthesis achieves the colour white by subtracting, not superimposing, the four basic colours.

In this way are we able to reproduce the entire colour spectrum?

No. Four-colour process printing manages to reproduces about 50% of colours visible to the human eye. Certain colours, such as orange, violet, and certain tones of green, are very difficult to reproduce with four-colour process. The graph below shows this. Notice that even the RGB process manages to reproduce only about 80% of colours visible to the human eye.

What do we do, then?

In the best of worlds, we print using a twelve-colour process. Professional photographers use photo printers that do this. But it’s very expensive. It’s a viable option for high-end photography printing, but not for books.

What do you think of software programs that suggest using Pantone colours?

I don’t recommend it. Since four-colour process only manages to reproduce 50% of colour visible to the human eye, such software programs will need to convert the Pantone colours to four-colour process before printing. This will inevitably create differences between what’s seen on-screen and the printed colour. If you’re looking to create a solid (also called flat colour), you’ll want to create your colours in four-colour process. This way the colour you create on-screen will match the colour you want to print, granted you have a properly calibrated computer screen.

Why do our eyes pick up certain differences in colour more than others?

Our eyes are very good at picking up slight differences in light tones, while they’re somewhat less apt in mid-tones, and quite poor in dark tones and shadows. If you have a high contrast image with many different colours, our eyes will have trouble noticing subtler contrasts in the image. In a low-contrast image, for example a black and white photograph or a low contrast photograph, our eyes will quickly pick up the subtlest contrast and variations. It’s important to note that we do not all perceive colours the same way.

High-contrast image with classical treatment
In this type of image a 2% variation in primary colours is barely discernible since our eye isn’t able to isolate the different colour zones.

Image featuring a 2% cyan variation

Image featuring a 2% yellow variation

B&W image with classical treatment
This B&W image has been treated in 4-colour process. The slightest variation (2% in this image) in any of the three primary colours is immediately picked up by the naked eye.

Image featuring a 2% cyan variation

Image featuring a 2% yellow variation

B&W image with GCR treatment
We chose this image precisely because of the challenge it presents, since it is mostly constituted of high-tone solids. GCR stabilization doesn’t offer a perfect solution, but the 2% variation in one of the primary colours is significantly less visible than in the images above, in which no GCR treatment was applied.

Image reproduced with GCR stabilization
and featuring a 2% cyan variation

Image reproduced with GCR stabilization
and featuring a 2% yellow variation

Let’s talk about printing systems.

Each printing system has its own advantages and shortcomings. There’s no perfect system in which you’ll have zero variation. We all know that colours don’t always display the same from one computer screen to another. It’s the same with different printing presses. Even different presses built by the same producer.

And in book printing?

The four-colour process that we use today has the advantage of being very consistent. It’s a good compromise if you’re looking to create high quality while keeping production costs low.

Design and printing processes have made great strides in the past two decades, thanks to the digitization and automation of adjustments that previously had to be done manually. Processes have become more standardized. But there still remain many parameters that will affect an image’s colour rendering:

• Paper colour can give a slightly warmer or cooler tone to an image. Graphic designers should use different ICC colour profiles depending on the paper that will be used to print. Though I’m aware this can be complicated when switching from one project to another.
• Offset and digital printing processes tend to produce the same rendering, but there can still be differences between the two, notably because offset printing involves a chemical process that isn’t 100% consistent.
• In offset printing, the paper’s chemical composition can, for example, produce a different reaction (emulsion) in the ink and in the water. Some papers tend to fluff, thereby introducing chemical elements that can disturb the balance of the damping circuit. This can affect colour rendering.
• Offset printing also uses an additive to stabilize the acidity of the water in its damping circuit and improve its surfactant properties. It’s a question of balance. If the water is too acid, you risk damaging the press. If the water isn’t sufficiently acid, the quality of colour reproduction will suffer. Changing the properties of the surface water film will also affect the quality of colour reproduction, as it will affect the film’s ability to de-grease the offset plate. It’s quite an art, as you can see.
• The processes used by digital laser printers are much more consistent. Here’s how it works: the base fabric for printing (similar to the offset blanket) is electrically charged according to the image to be reproduced, the powdered ink being charged with opposite electrical charges. The paper is then heated so as to transfer the ink particles. The press’s internal temperature has to be regulated, and depending on the day the paper may contain more or less humidity which will affect its temperature. This can create slight variations in colour reproduction. But the main reason for colour variations on digital presses is the fact that each press is calibrated differently. Different machines use different technologies. Here at Rapido we use the best press on the market for colour printing: HP Indigo. It’s important to add that press operators have a fine-tuned eye, but they don’t all perceive colours exactly the same way. We could also mention ink-jet printing, though colour is not yet its strong suit.

It’s complex!

Yes, but there are a few good practices you can follow to avoid the most common problems. When you make sure to provide print files that meet professional standards and make sure to work with a good printer, things will go much more smoothly.

Can you give some examples of good practices?

A little over thirty years ago, colour expert Felix Brünner established rules for colour reproduction. Since then, Brünner’s rules have been followed by thousands of printers worldwide. He’s one of the references when it comes to colour reproduction.

Here are a few of Brünner’s key rules:

• The easiest images to reproduce using four-colour process are high-contrast images, because the human eye doesn’t readily notice differences in a high-contrast image.
• When an image contains continuous tones (for example, a uniform background or a face) and the composition values range between 1 and 35% (light tones), it can be difficult to ensure consistent results because the eye will notice even the slightest variation.
• When these continuous tones are composed of the three primary colours (cyan, magenta, yellow), colour reproduction becomes even more unstable since there will be variations in all three colours. Skin tones belong to this category. The slightest variation (+/- 2%) in any of the three primary colours will result in a face being visibly too red, not red enough, slightly violet, greenish, etc.
• The most unstable colour images are those that contain a dominant grey (jewellery, for example). The same goes for solids with a trichromatic composition, since these contain similar values of all three complementary colours. The eye will catch the slightest variation in such images. If the cyan is 1% below the theoretical value, while the yellow and magenta are 2% above, you’ll end up with a grey balance that’s noticeably too red. And you’ll think it was a printing error, while 2% remains well within tolerable variation in the printing industry.

Do you have any recommendations?

Felix Brünner recommended that photoengravers stabilize their images using the UCR mode (undercolour removal), which involves substituting the three complementary colours with black in areas of the image where these colours have equivalent values. There’s also the GCR mode (grey component replacement), which is better adapted to images containing dominant greys. GCR involves substituting an image’s complementary colour with black. I recommend it for photos of jewellery, as well as black and white photos printed in four-colour process. Your images will be more consistent and any variations in the printed image will be practically imperceptible.

You seem to have quite a bit of experience in this field.

I ran a photoengraving studio for ten years, and a large offset printing plant for fifteen years. My teams and I produced millions of pages of colour advertising, and at times we inevitably ran into issues. Every time we were able to convince our clients to adhere to Brünner’s recommendations, we noticed that there was significantly less variation. We even managed to reduce our overall ink consumption by 10%. We can summarize Brünner’s thought by the following statement: use a bit less ink to print better.

Why do designers like to use trichromatic greys so much?

Because it’s pretty! It looks great on-screen, but it’s difficult to reproduce in print. When you create a solid using values like Cyan 10% + Magenta 8% + Yellow 5%, you can be pretty sure that your solid will vary from one print run to the next. You might even notice variations within the same print run, without it being the printer’s fault. The lower the composition values, the more this problem intensifies because our eyes are attuned to slight variations in light tones.

How should we be composing our solids?

Substitute the complementary colour for black, as in GCR. You’ll need to adjust the values of your two other colours afterwards, which will require a bit of testing and research. But by doing this you’ll stabilize your colour and even make it livelier.

By adding black?

It’s surprising, isn’t it! Try it and you’ll see. At Rapido we work with a team of over ten designers who have adopted this technique. Today they all find that the colour rendering in print is much closer to what they see on-screen. In a nutshell, the paler your colours are, the more I recommend that you add black. I swear it’s the thing to do.

What advice would you give to a designer who is interested in colour reproduction?

If you want to make grey, use only black. If you want a slightly warmer grey, add a bit of red. For a cooler grey, add a bit of blue. Otherwise, contact me. We’ll run some tests if you want. We do this for free. We’re always learning, even after thirty-five years in the business. And the more we collaborate, the higher the quality of our product will be.

References:

Read more about Brünner’s colour theory: https://www.systembrunner.com/basics-knowledge/picture-contrast-theory