Colour is one of the most critical assets within a designers toolkit. Colour allows us to create epic visual communication full of expression and emotion. However, when it comes to print, colour is also an area that requires some technical understanding to ensure that results match expectation. GSM takes a look at Colour – CMYK, RGB and PMS…
CMYK Colour
Most commercial print processes are based Cyan (C), Magenta (M), Yellow (Y), Black (K) colour model—otherwise known as CMYK or ‘Process’ Colour.
Here’s how it works: If you did art at school, you likely did an exercise where you mixed Blue, Red and Yellow paint together in varying amounts. Mixing these ‘Primary’ colours creates all other colours—i.e. blue + red makes purple, blue + yellow makes green, red + yellow makes orange. Adding Black makes colours darker. Adding white makes colours lighter.
CMYK Colour is based on the same basic principle: In fact Cyan is a Blue, Magenta is a pinky type of Red, Yellow is Yellow—so essentially, CMY is a specific set of the Primaries. The addition of Black (the K in the acronym CMYK stands for ‘Key-Black’) adds more depth. Collectively, these four colours are known as the Process Colours. Within CMYK-based printing, the Process Colours are used to create all graphics and photographic images.
CMYK Colour Formulas
To make all other colours using CMYK, each of the four Process Colours are printed as either 100% solid (meaning total colour coverage at full density), or as a tint ranging between 1—99% (meaning less colour coverage), or not printed at all (0% means no colour is laid down). Looking at the image above; you can see how each of the process colours starts as a solid and reduces down in ‘tints’, making the colour lighter until we reach ‘white’. In CMYK printing, white is not a printed colour—merely the absence of printed colour (i.e. the paper colour). It is through overprinting these solids and tints on top of each other—that all other colours are created.
The Colour Wheel
In the above colour wheel, we can see how various tints of Cyan, Magenta and Yellow create other colours. By adjusting the relative tints, removing one or more of the Process colours, or by adding Key-Black, we can make any colour lighter, deeper, or the hue shifts entirely.
Combinations of “Red”
The examples below show a ‘red’ made from 100% Magenta plus 100% Yellow. And how this colour combination changes by adjusting the formulas:
A. Shows the original formula: Solid Magenta plus solid Yellow equals red.
B. Shows the same colour combination with Magenta and Yellow halved to 50% tints—the colour is lighter.
C. Shows the original formula, but with a 50% tint of Key-Black added— the colour becomes darker.
D. Shows the same colour combination with Magenta reducing to a tint of 50%, Yellow remains solid—the hue shifts from red to orange.
CMYK Formula Sequences
All colours printed using CMYK are created according to this methodology. In CMYK colour, these combinations are commonly notated as a formula sequence (also called a colour breakdown) which shows a percentage between 0 and 100 for each of the four Process Colours. These colour breakdowns are commonly notated according to the following convention:
C=0 M=100 Y=100 K=0
Reading the above formula—no Cyan or Key-Black will be printed, but Magenta and Yellow will both be printed as 100% solids—this formula matches our example ‘red’. Within CMYK-based printing, these formulas are the correct nomenclature to describe an exact colour. These formulas can be used as colour swatches within industry-level design apps such as Adobe Illustrator and InDesign
CMYK Halftone Screens
The key to understanding colour theory for print mediums is knowing that within CMYK-based printing—all colour is created using solids/tints, or the absence of these—in each of the four Process Colours. But how does this work?
In any form of CMYK-based printing—each Process Colour is printed as a series of tiny dots that form a halftone screen. The size of the dots (which vary), and the spacing between the dots (which allows more of the paper surface to show through), creates the printed effect of a tint. These halftone dots are so small they are perceivably invisible to the human eye. (You can only see these using a magnifying glass or an eyepiece).
How Halftone Screens Work
The image below shows how different halftone screens work:
The four examples in the left Column show Cyan as:
A. 100% Cyan or C=100 M=0 Y=0 K=0: When magnified, this colour is fully solid, there is no paper show-through.
B. 90% Cyan or C=90 M=0 Y=0 K=0: When magnified, this colour comprises a series of dots which are mostly overlapping. Where the dots do not fully overlap—the paper shows through. Thus creating the illusion that this is slightly lighter than solid Cyan.
C. 50% Cyan or C=50 M=0 Y=0 K=0: When magnified, this colour comprises a series of mid-size dots. Half the colour area is actually the paper showing through.
D. 10% Cyan or C=10 M=0 Y=0 K=0: When magnified, this colour comprises a series of very small dots. Most of the colour area is actually the paper showing through. We see this as being almost white.
Overprinting Halftones
Overprinting halftone screens of varying sized and spaced dots (including solid areas)—creates all colours in CMYK-based printing. Note that the dots themselves, within the halftone screens, are printed as 100% solid ink—no actual tints of ink are printed—it is the varying size and spacing of the dots that creates the illusion of tints and gradients. You can see this in the following example:
This CMYK grass green (C=50 M=10 Y=100 K=10); is comprised of four halftone screens. When magnified you can see how this colour is simply screens of Cyan, Magenta, Yellow and Key-Black—when overprinted (printed one colour over the other) these create green.
Applying this theory to a design:
The below image shows how an A5 flyer (148mm x 210mm) is created using four halftone screens—one for each of the Process Colours. Each of the halftone screens, if magnified, is simply a series of varying sized and spaced dots (including solid areas)—which are overprinted to create the final result.
The Limitations Of CMYK-Colour
CMYK colour is used to produce most commercially printed material. However, it is important to understand that CMYK has inherent limitations and is not the only way to print colour. In fact, we can only print a portion of the visible spectrum using CMYK. In particular, hues such as oranges, violets, emerald greens and deep reds have a tendency to flatten off, lack saturation or go muddy. This may not be particularly obvious when looking at photographic images as the variation of colour, within these images, disguises this. But can be quite obvious in solid areas of colour.
Non-CMYK Specials & Spot Colours
The term ‘specials’ covers a range of ways to create colour that are not CMYK-based. It is important to note that these specials vary based on the print method being used (as explained below). However, regardless of process, these specials are most commonly printed as ‘Spot’ areas of colour—meaning, applied as fills and strokes within Vector elements. We seldom use Spot Colours to print photographs (although this can be achieved in some instances). There are a couple of key reasons why you might use Specials instead of, or in addition to, CMYK;
- Specials facilitate printing colours beyond what CMYK can create.
- Unlike CMYK where colours can vary—Specials create consistent matches.
Here are some common types of print Specials:
Pantone PMS ‘Spot’ Colour
The most common form of ‘Spot’ colour is Pantone PMS (Spot) Colour.
Pantone Corporation (pantone.com) is an American company that develops colour systems used by several creative industries, including graphic design and printing.
The specific system for printing is the Pantone Matching System (commonly abbreviated to PMS or often referred to as ‘Spot’ colour). PMS colours can be used in addition to, or instead of, CMYK on commercial ink-based print processes: Offset Lithography, Flexography, Rotogravure and Letterpress. It is important to note that (at the time of writing), the Pantone Matching System is not used by any form of Digital Printing—but some digital systems do offer their own ‘specials’ (see Digital Specials below), but these are not Pantones.
The Pantone Matching System is based on a series of swatchbooks used to specify colours. Each Pantone colour is identified by a unique PMS code which, in most instances, is expressed as the acronym PMS followed by either three or four digits to identify the individual colour, plus a C or U suffix—indicating whether the colour has been specified from the Coated or Uncoated swatchbooks (see below). An example Pantone Colour is: PMS021 U—an orange.
Printing Pantone
In addition to identifying and specifying colours, the printer uses these codes to reference and mix the Pantone colour. Pantone colours are mixed ‘off-press’ from a series of base inks (a process not dissimilar to mixing house paint), the mixed ink is then added to the printing press.
You can use one or more Pantone colours on their own, or in addition to CMYK. For example CMYK + one or two PMS colours (a five or six colour job). On a large multicolour Offset Lithographic, Flexographic or Rotogravure press—a five or six colour job is usually printed in one pass with the Pantone colours going down at the end of the sequence after the Process
colours. Note that Letterpress printing tends to use Pantone colours (or other Spot colours) only, or sometimes straight Key-Black—CMYK Process Printing is not common with Letterpress.
Pantone Swatchbooks
Supporting the Pantone Matching System is a range of swatchbooks (see below). Most Pantone Swatchbooks come in both a Coated (C) and Uncoated (U) version.
>The difference is the paper on which the swatchbook has been printed—note that the colour breakdowns are the same in both versions. Coated swatchbooks are printed on Coated
gloss board, Uncoated swatchbooks are on an Uncoated board. The reason for two versions is because the different paper stock affects how the colours appear—colours printed
on uncoated paper are generally less vibrant. This means when colour specifying using these swatchbooks—make sure to use the correct version. The only exception is the Metallics
Guide, which comes in a Coated version only, as these inks do not tend to print well on Uncoated stocks.
The different Pantone Swatchbooks:
1. Formula Guides: Two x books (C & U): Formula Guide swatchbooks show the base range of 2,400 PMS colours, along with the corresponding ink formula. These guides are primarily used by Printers to mix Pantone inks.
2. Bridge Guide: Two x books (C & U): The Bridge Guide swatchbooks show the same range of colours as the Formula Guide—but instead of showing the ink formula, the Bridge Guides show each Pantone along with its nearest CMYK equivalent—plus provide the corresponding colour breakdowns (including CMYK, RGB and the Hexcode). The Bridge Guide swatchbooks are great tools for Graphic Designers doing colour specifying. If you are only going to own one set of Pantone swatchbooks—this is the one to have.
3. Metallics Guide: One book—Coated only: The Metallic Guide contains 600+ additional colours not featured in the Formula/Bridge guides—all of which use a metallic base such as gold or silver.
4. Pastels & Neons Guide: One book (C & U): The Pastels & Neons Guide (combining Coated and Uncoated within the same book) shows 150+ pastels and 50+ neons— not featured in the Formula/Bridge guides.
5. CMYK Colour Guide: Two x books (C & U): Show CMYK colour formulas—not PMS colours. These can be a useful tool for specifying and checking CMYK colours.
For more information about how paper affects colour— refer GSM blog – Paper.
Digital Specials
‘Digital Specials’ is a generic term encompassing a range of print effects offered on various digital print systems. These ‘Digital Specials’ tend to be printed as Spot Colours in the same manner as Pantone PMS Colours (but are not PMS colours). It is important to note that Digital Specials vary from system to system (there is no singular standard). Some systems offer metallics (such as a gold or silver). Or other colours such as white, fluoro or boosting colours such as an orange, blue or green. If you want to use Digital Specials ask your printer what their system offers.
Non-Printed Specials
And, in addition to print-based specials—there are also non-print specials, which are done as separate processes after printing. These are covered in detail in GSM Blog – Beyond Print.
Screen Colour – RGB
Previously, we discussed that not all colour was possible using the CMYK colour model. Going one step further, it is also important to understand that colour for commercial print—regardless of how the colour is produced— has significant limitations when compared to colour on a screen. Understanding why this occurs can help you navigate this issue to avoid the frustration of printed colour not matching what you are seeing on screen.
Colour on a screen is created using Red, Green and Blue light (otherwise known as RGB). This light is projected using pixels—the basic physical unit that comprises a digital screen. Each pixel comprises a series of Red, Green and Blue diodes. These emit varying amounts of light in these colours, ranging from zero (no light) up to 255 (bright light). On a practical level, it can help to think of RGB as doing the same thing on a screen, as CMYK does in print. It is the combination of these RGB colours that creates a colour spectrum on a screen.
The key difference between RGB and CMYK is the size of the respective colour spectrum. RGB can create more than 16 million colours. This is significantly more than what we can actually distinguish (estimates are that humans can perceive about a million colours). And vastly more than CMYK can produce on a practical level. Which brings us to the core of this issue—print has a vastly reduced colour spectrum compared to colour on screen. It is this mismatch between the CMYK and RGB colour spectrum that results in many colours printing differently to what we see on screen.
Exercise: Print vs Screen Colours
Understanding the differences between RGB and CMYK colour spaces is crucial for a designer working within print. These two colour spaces are mismatched, with RGB having a much larger colour spectrum than CMYK. You can see this first-hand by trying this exercise:
- Create a new file in Adobe Illustrator or Photoshop using one of the web presets (any physical size / resolution is fine)—but ensure the file is in RGB colour mode.
- Create a colour fill using this formula):
R=0 G=220 B=0 (#00dc00) - Now compare the colour in your screen file to the printed sample shown above. This is the closest match CMYK can achieve to this RGB colour. As you can see, this is not particularly close.
- Back in your file: if you change Colour Mode from RGB to CMYK you will see this colour shift on screen:
> Illustrator go to: File > Document Color Mode > CMYK
> Photoshop go to: Image > Mode > CMYK Color
Changing the Colour Mode of a file from RGB to CMYK forces any RGB colours to be reinterpreted to their closest CMYK equivalents—mimicking what will happen at the printing stage. This is the primary reason for working in CMYK Colour Mode instead of RGB Colour Mode for print artwork. The colours you see on screen are significantly more accurate using CMYK Colour Mode (note—these colours are still not 100% accurate).
CMYK Colour Methodology
Using a CMYK-based method to print an RGB colour requires that the RGB colour be reinterpreted as a ‘nearest’ approximation. You can, however, avoid this situation through how you approach colour when working towards a print outcome.Here are some best practice tips:
- Remove RGB from your work methodology for print artwork. This means only using CMYK colours within designs, and only using CMYK Colour Mode for artwork files—do not use RGB for anything print-based (note; this includes all linked images and graphics—make these CMYK).
- Do not not choose or specify colours for print projects based on screen representations. When choosing colours for print, specify these by using a printed reference such as CMYK Colour charts (at the back of this issue of GSM), or the Pantone Bridge or Pantone CMYK Colour swatchbooks.
- Always ask your printer to run a hard copy proof from the final artwork. A printer’s proof is the most accurate method of checking how the colours will print. If the colour on the proof is not what you expect—then you need to fix this before the project runs.
