When is it appropriate to use black in non-gray colors using CMYK?

Question

I am learning to create documents under a CMYK color model.

Unlike simpler color models such as RGB, CMYK facilitates the expression of similar colors through very different combinations of base colors, because shade can be achieved either through adding black, or by adding the other colors.

An essential reason, as I understand, in modern printing, for including black among the inks is that black text and other typographic elements show poorly if not represented by black ink. Thus, without a specific reason to use a "rich black", black ink for black typographic elements is the obvious choice.

In general, in which other cases is any use of black ink appropriate? When designers produce a line, fill, or text on a gray scale, should black ink be used to the exclusion of chromatic inks? When designers produce a line, fill, or text that is not black or gray, should chromatic inks be used to the exclusion of black? In which, if any, normal cases should black ink be mixed with chromatic inks (e.g. cyan, magenta and yellow)?

Answer 1

Using black ink for non-gray colors is very common. Nothing wrong in that.

When you convert an image from RGB to CMYK, you normally trust the color profile to find the CMYK numbers needed to reproduce each pixel as accurately as possible. Most standard color profiles will use black ink throughout the image, also in the more colorful parts which could be reproduced without black ink. This is done in a pretty complex compromise between color likeness, stability of tone, total ink, smooth transitions etc.

When it comes to color swatches for vector objects, most designers like to "program" their swatches directly in CMYK. For many everyday jobs we could as well define our colors in RGB or Lab (while making sure to keep them inside CMYK gamut of course) and just convert them to CMYK on export and thereby leave it to the color profile to decide on the specific CMYK numbers. This would make it possible to export to different CMYK profiles for different paper types. Actually, defining swatches in CMYK is often the same as working in RGB. You might nudge the CMYK values up and down, but you rely on the RGB preview on your screen to decide if you like the chosen color.

All this said, there are still reasons to want to control the CMYK values manually. Keeping black text and line art exclusively black (preferably at 100% to avoid halftoning) and making sure neutral colors only use tints of black ink are some of them. Here I'll show some other examples where it can be wise to consider exactly how to define the CMYK values. The examples both speak for including and excluding black ink in colors under different circumstances. That's the complexity of it.

(In the following I'm using the color profile ISO Coated v2.)

Minimize Unwanted Color Tint

As you already know, the same color can often be defined with or without black ink.

Below are two colors: some kind of curry yellow and a warm gray.

The left version of each color is defined without cyan ink and the right is defined without black ink.

Let's imagine they were printed on slightly uncalibrated equipment which resulted in a shift of for example CMYK(+5,-2,-5,+5).

Notice how the colors to the left (without cyan ink) mostly get a change in lightness, while the colors on the right (without black ink) also get a visible shift in hue.

The "tinted grays" are the most sensitive. If I want to make for example a yellowish gray, I would never add anything besides yellow and black ink. Why risk getting a greenish or reddish tint by adding cyan or magenta ink if it's not needed to obtain the wanted color?

Smooth Transitions

When you want a smooth transition between two colors, for example in a gradient, you get the most aesthetically pleasing result if the two colors share one or more inks.

Let's assume we want to make a transition from some dusty blue to a pure 100% black.

To the left the blue is defined without any black ink. The gradient becomes pale in the center because all of the inks of both colors must fade all the way to zero.

In the middle the blue is defined with some black ink. The transition becomes slightly better as the black is present all the way through the gradient.

To the right I've converted the two colors from the leftmost gradient to Lab and back to CMYK again. Now the color profile decides the CMYK numbers and it adds color to all four inks. The transition is much nicer, but the black color is no longer just 100% black.

So if the purity of the black ink is important (for example if it has to blend together with other graphics or it contains negative text), the middle solution is probably best. If the appearance of the gradient is more important, the solution to the right might be best.

Here is another example with the same principle, only this time it goes from that same dusty blue to a green color.

Here it seems like the left solution without black gives the best result. The middle one looks a little bit faded in the center. The right one looks fine, but in this case there is probably no need to add black to complicate things unnecessarily.

Cleanness

Two CMYK colors that apparently appears exactly the same on screen (has the same Lab values) will in reality look a little different on print texture-wise.

These to colors looks very similar on screen although they are defined differently.

Seen from a distance or when squinting your eyes, they will look the same, but a closer inspection will reveal that one of them has tiny black halftone dots while the other hasn't.

It might be a subtle difference, but in my opinion the black dots do pollute the color a tiny bit and it might look cleaner and less flickering to avoid black here.

Avoid Darkening of Colors

Even though most print houses follow a standard there is still some craft to it. The printer might want to make small adjustments while printing to get the best result.

If you for example make a print with black text, grayscale images and some colored areas, it might be a good idea to define the colors without black ink. This way the printer can turn up the black a tiny bit to get darker text and images without affecting the colors.

Minimize Misalignment

It's recommended to keep small text in 100% black to avoid misalignment issues. But sometimes you just can't avoid having small colored text. In those cases it's best to define the color of the text with as few inks as possible.

Here are two texts defined in two inks and in all four inks.

On print the inks might get slightly misaligned and the text with only two inks looks a little less blurry.

Eliminate Misalignment

Sometimes you want detailed graphics in one color on top of another color. This can lead to problems with misalignment. Those problems are countered by trapping, but it can still look messy. With a little attention to the CMYK values, you can eliminate the misalignment completely.

Look at this green square with an orange cross.

In the left version the only difference between the two colors is the cyan ink. In the right version the green color is the standard mix of all four inks and all the four inks differ.

On print these two approaches will look rather different.

The left one can't be misaligned as the shape of the cross is only made with the cyan ink. The right one needs to be trapped and can look a little blurred.

Answer 2

It's difficult to provide such a clear cut, consistent, answer to this the way there's one for type.

In general, the reason it's preferable to use only black for text is so that mis-registration doesn't reduce readability or create undue difficulty with registration. With type, any mis-registration can cause a sense of "blurring" which can reduce readability of smaller text.

If including solid line art, containing hairlines or minute details, it can be advisable to use only black ink for the same reasoning. If there's no benefit to a color build, there no reason to include it since it can only lead to "blurring" if there's any mis-registration.

Greyscale art may benefit from a color build, much like rich-black is beneficial. It's only when something is small, detailed, that the lack of the other color plates can be helpful. Much of this depends upon the artwork. If you are seeking a small 1pt or smaller rule, which appears to be 50% K there may not be a great deal of benefit to a rich black for such an item. On the other hand a 5pt rule which appears to be 50% K may benefit from a color build (rich grey).

Solid black can also easily be overprinted making registration much less of an imperative. This, in turn, makes the pressman's job less troublesome... so that's another reason print providers will prefer only black if the artwork supports its use.

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As for excluding black... well that goes more to the desired appearance. If you want a color that is strictly Cyan, then you'd benefit my creating strictly cyan artwork with no black. But how often do you want strictly Cyan? The other plates are not the same as the black plate.

Answer 3

In my opinion, this is an excellent question.

Without a specific reason to use a "rich black", black ink for black typographic elements is the obvious choice.

Yes, the main reasons are two.

1. Avoiding possible problems of registry, this is two or more inks not perfectly aligned. On small elements, this misalignment is more notorious than on large elements. One misalignment of, let's say .5 mm on a big square could not be visible, but on typography is.

2. To properly print a rich black you need different percentages of inks, which, in turn, will have dots due to the screening, This will render fonts fuzzy.

When designers produce a line, fill, or text on a grayscale, should black ink be used to the exclusion of other colors?

Follow the same two principles, if you have a thin line you can potentially have a misalignment or a fuzzy finish.

that are not black or gray

You are "approaching" to another reason.

3. You can make a gray using CM & Y, but any variation on the density of the ink on any of the 3 plates will shift the neutral gray to a tint to some degree.

In general, in which other cases is any use of black ink appropriate?

Some cases come to my mind.

A duotone normally uses black and other ink.

Probably some gradient from dark to bright needs black to be used in a controlled fashion.

Large gray areas are easier to control using only 1 plate than 3.

I will only mention the different cases of rich black, which needs an understanding of the usage of black.

A. When you use black and another plate to give it more depth and overall temperature.

B. When the rich black (or grays) are defined by the color profile.

Another technical usage is that you need to understand how overprint works, in some cases, the black can have overprint enabled and in some other cases, you do not need it. One example is black text over a colored background.