9

I don't understand how black can be added in CMYK color mode. So green is made up of 85% cyan, 14% magenta, 100% yellow and 2% black. But I thought black is created by combining 100% of cyan, magenta and yellow, so where does the 2% come from?

  • 10
    In color theory 100% C + 100% Y + 100% M = black but in the real world 100% C + 100 % Y + 100% M = brown. Anyone who paints or use color pencils know this – slebetman Oct 18 at 18:44
  • 1
    @slebetman in the days of 3-colour desktop inkjets, composite black was dark green. I printed lots of posters on one where you could have a 3-colour cartridge or and black cartridge. You couldn't do colour separation into CMY on one run and K on the other unless you designed in big gaps: the registration wasn't good enough (the software didn't help much either). – Chris H Oct 21 at 10:45
  • @ChrisH CMYK didn't come from colour printers. The CMYK color format was designed for color separated offset printing. On old color printers without black you could not get real black (indeed, some early black and white inkjet printers could only print dark grey/slate at best and not black - you have to send your print to a real printing press to get magazine quality blacks) – slebetman Oct 21 at 16:33
  • @slebetman indeed, but it's necessary for colour inkjets too. For many of us that's the first time we came across CMYK, and a cheap inkjet is still the best chance for playing with mixing (especially if you've got a microscope) – Chris H Oct 21 at 17:46
  • @ChrisH Yes, but the question is where does the K comes from - implying that the OP mainly only knows about CRT/LCD color model. It doesn't come from monitors/displays and it doesn't come from inkjets - it comes from offset printing which predates electronics (analog or digital) much less computing – slebetman Oct 22 at 5:08
19

It comes from:

1. The imperfection of inks

An ink is made from available materials, which need to be affordable, safe, reliable, etc. and they need to give reasonable results.

With the current technology, the inks are a good compromise on those points, but they are not perfect. When we combine 100% of Cyan, magenta, and yellow, they do not manage to absorb 100% of the light so the obvious choice to make things look black is to add black.

2. It is a good old friend

Let us add some additional characteristics of black ink. It is probably a good old color we are used to having on a book, since way before they were printed books, manuscripts, ever since cave paintings. It is stable, good for sharp text, etc.

3. It is cheaper in the long run

Black ink is cheaper to produce, so it is easier to use when we actually need black.

So in the current CMYK model when we need to darken a color that is first darkened with some complementary colors (1), we start adding black to the mix little by little until the dominant color on a black image is black ink. Some years ago this was a method of saving costs when reproducing color prints, trying to replace the neutralized CMY colors as soon as possible, which was called achromatic method. Now days is more important having nice vivid contrasting colors, but it is still cheaper adding black.

4. So adding the above, it is simply logic to use it


(1) Answering your second question.

Black starts to replace neutralized colors. When you have two primary colors, Cyan and Yellow; Green in your example, it has the "maximum" saturation for that color (forget for a bit the lightness of it).

When we start adding the last primary color, two main things happen, it starts to darken, and it starts to be neutralized moving it to black.

Adding black too early on light colors will be too abrupt, so neutralizing it with only the complementary is a good option (Some Magenta). When it is dark enough that this black ink does not show a clear border, we start replacing the color with Black.

Modern inkjet printers even have some light versions of Cyan and Magenta, others have lighter versions of black, so these dots and transitions are more subtle in light colors.


Let me spam you with some examples I made. The page is in Spanish but just look at the images. Here it is how a perfect CMYK ink would transition. And here is an example of how black is introduced little by little when color gets darker.

  • 1
    "lighter versions of black" is a new concept for me. Is it similar to "lighter shade of pale"? – James K Oct 18 at 21:47
  • 2
    :o) Yup, normally called gray. – Rafael Oct 19 at 3:48
  • 2
    @JamesK: In practice, no “black” items are ever true black. The black ink in a newspaper is not as dark as, say, any black plastic items you have in your kitchen, which in turn are not as dark as a black night sky. (Go somewhere with little light pollution and a good view of the sky, on a dark night; hold up a matte black item against the sky, and point a flashlight at it. It will be clearly lighter than the sky.) In short: there are plenty of lighter and darker shades of black; “black” pretty much just means the darkest grey we’re considering at a given moment. – Peter LeFanu Lumsdaine Oct 19 at 17:51
  • 1
    @PeterLeFanuLumsdaine human eyes are adaptive, we are not even capable of absolute measurement. Black does not even need to be a neutral color just as long as its the darkest color and there is a color cast. This is why you do such things as black and white point correction when transfering between mediums. – joojaa Oct 20 at 5:55
  • @Peter Anything painted Vantablack (or that new black accidentally created at MIT which is even blacker) comes pretty close to being true black, though. And if you count a black hole as an ‘item’, that should count as absolute, true black. – Janus Bahs Jacquet Oct 20 at 10:16
16

C = Cyan
M = Magenta
Y = Yellow
K = Black

B was not used as an abbreviation because it could be confused with "blue" from RGB, or "blue/yellow" from L*A*B

In addition to being an excessive amount of ink.....
100C/100M/100Y will yield a dark brown, not black.

  • 6
    FWIW, Also, K was a "Key" to registration. In the absence of a black plate, the darkest colour is used as the registration key. The key was often the first down so that lighter colours could be more easily aligned (visually) to it but wouldn't be so obvious if misaligned. Now, it is more common to use registration marks for precise mechanical alignment and the "key" (black) is often the last down layer. – Stan Oct 18 at 14:41
  • Yes. That's how K was determined to be a good alternative to B :) – Scott Oct 18 at 19:58
9

I have added this answer, although it covers what has already been said by others here, to give you a visual aid to understanding the issues.

Green doesn't need to have black or magenta in it. These have been added to darken the green.

100% CMY does not make black. It makes a dark brownish/muddy grey because as Rafael explained, the inks don't absorb all the light. Add to this the fact that CMYK inks are actually semi-transparent anyway, and you see the problem. To get a darker black you need to add black into the colour. Black ink alone does not make a dark black either. To get a "rich black" you also need to add CMY inks.

The examples below show your green in the middle of the top row. To the left is the same green without the black, and to the right the same green without the magenta and black. The bottom row left shows 100% CMY compared to a rich black in the middle, and to the right black only without any CMY inks.

enter image description here

3

Adding to the answers by others using (printing) industry terms:

While it's true that C, M, & Y can be used in varying proportions to simulate full colour in a printed image, there's a limit to the amount of ink that can be put on a page.

100% Cyan, 100% Magenta, 100% Yellow (solids) ink coverage won't dry.
That's three layers of ink on top of one another.

TAC (Total Area Coverage) From Colour Quality Consultancy

Typical TAC values for coated stock are: sheetfed offset 300-340%; heatset web offset including SWOP (US Specifications for Web Offset Publications) 300%. uncoated stock is usually considerably less. Uncoated newsprint on non-heatset web 240-260%.

What to do?

Since this situation is a layer of black, effectively, some bright person wanted to know, "Why use three layers of expensive matched process colour inks when we could accomplish the same thing with one layer of black which is much cheaper, looks better, and dries faster?"

No one could think of a good reason so UCR was invented.

UCR (Under Colour Removal)
Whenever it's possible to substitute a black for equal amounts of process colour inks, do it.

A few jobs later, someone noticed another time and money saver.

GCR (Grey Component Replacement)
In addition, whenever a third colour is needed, black is used instead since it (in effect) adds black.

2

You've gotten a few answers explaining why black is used. But let me take your question literally:

Where does black come from in CMYK color mode? […] I thought black is created by combining 100% of cyan, magenta and yellow, so where does the 2% come from?

The black color in CMYK is not made by combining cyan, magenta and yellow. It's made out of carbon:

A pile of carbon black pigment, in the form of powder.

(Image from Wikipedia Commons.)

I don't understand how black can be added in CMYK color mode.

Remember that the CMYK color space is based on actual, physical ink. C means cyan ink, M means magenta ink, Y means yellow ink, and K means black ink. So, "2% black" quite literally means "put a small amount of black ink on the paper." That's how black is added.

  • 1
    I think OP doesn't mean literally where does the black come from, but they didn't understand the concept of CMYK where K is the black ink. Good info nonetheless. – Luciano Oct 21 at 9:11
  • 3
    @Luciano Well, the asker states that they thought that in the CMYK model, black was simply a combination of cyan, magenta and yellow. I thought it would be useful to explicitly point out that this is not the case, and I don't see any other answers which clearly say that. – Tanner Swett Oct 21 at 21:35
0

Black pigments are cheaper than color pigments and have very good absorption across the spectrum. The CMY pigments only absorb a part of the spectrum and the overlap of the absorption curves is not perfect. For that reason, substituting black (K) for the common parts of CMY saves money and ink and improves the color representation. If one is aiming for comparatively precise colors under different illuminants, it turns out that substituting CMY with K isn't exactly equivalent, so one RGB color may be represented by more than one spot color in CMYK and not look identical under all illuminations even when the full darkness of black ink is not required.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.