Four color printing is a system in which a colour image is separated into 4 different colour values (called colour separation) through the use of filters and screens. This used to be done with photographic film in a graphic arts camera, but is now often done digitally with software.
The result is a colour separation of 4 images which, when transferred to printing plates and printed sequentially on a printing press with the colour inks cyan (blue), magenta (red), yellow and black (the k in cmyk), reproduces the original colour image. Most of the colour spectrum or gamut is reproduced with only the four process ink colours.
The process of four color printing is universally used in the graphic arts and commercial printing industry for the reproduction of colour images and text.
Why is black ink used in CMYK printing?
If cyan, magenta and yellow inks are mixed, we can obtain the colour black.
But this black colour generated by mixing cyan, magenta and yellow inks does not produce satisfactory results, so in four color printing, black ink is used in addition to the subtractive primaries. The most common reasons for using black ink are as follows:
- In the traditional preparation of colour separations, a red keyline on black line art marked the outline of solid or tinted areas. In some cases a black keyline was used when it served as both a colour indicator and an outline for printing in black, as the black plate usually contained the keyline. The K in CMYK represents the keyline or black plate, also sometimes called the key plate
- Text is usually printed in black and includes fine details (such as serifs), so to reproduce text or other finely detailed outlines, without slight blurring, using three inks would require inaccurate registration.
- A combination of 100% cyan, magenta and yellow inks soaks the paper in ink, causing it to dry more slowly, leading to bleed-through or (especially on cheap paper such as newsprint) weakening the paper so much that it tears.
- Although a combination of 100% cyan, magenta and yellow inks should, in theory, fully absorb the entire visible spectrum of light and produce a perfect black, in practice the inks do not achieve their ideal characteristics and the result is actually a dark, murky colour that does not look quite black. The addition of black ink absorbs more light and produces much better blacks.
- Using black ink is less expensive than using corresponding amounts of coloured inks.
- When a very dark area is desired, a CMY “layer” of colour or grey is applied first, and then a layer of full black is applied on top, making a rich, deep black; this is called a rich black. A black made only with CMY inks is sometimes called a composite black.
The amount of black to be used to replace the amounts of the other ink is variable, and the choice depends on the technology, paper and ink used. To decide the final mix, processes called undercolour removal, undercolour addition and grey component substitution are used; depending on the print job, different CMYK recipes will be used.
What other colour modes are there besides four-colour?
In addition to the CMYK colour mode used in four color printing, there are other types of colour modes such as RGB and PMS. It is important to understand that CMYK, RGB and PMS are different from each other. CMYK is the colour mode used in full-colour image printing.
RGB is the colour mode used in desktop monitors, digital cameras and scanners and can be used by large format printers (such as canvases). RGB stands for Red, Green and Blue.
PMS colours or Pantone Matching System or spot colours are specific colour formulas that will be accurately reproduced in print. Each PMS colour is assigned to a specific standardised number. PMS colours are pre-mixed whereas CMYK and RGB need to mix their colours to obtain a specific colour.
So it is also important to understand that there is a big difference between CMYK and RGB. By default, CMYK uses a white background (papers, blank canvases, etc.), as it is used in printing, while RGB uses a black or dark background, as this colour mode is used in computer monitors and scanners.
There are times when a specific colour cannot be produced by the 4-colour printing process and one of the solutions to this type of problem is to incorporate a specific PMS colour into the CMYK colours. This is known as 5-colour or 6-colour projects. As the name suggests, it uses the CMYK colours plus a specific PMS colour to produce a certain type of colour. It is often used for things like brand colours or company logos.
It should be noted that RGB colours cannot be used for commercial printing. The computer monitor is ideal for viewing the RGB colour mode.
So if you are going to use RGB colours to print full colour images, the end result will not be identical to the image on the computer monitor. There will be slight changes in colour shades as RGB and CMYK vary from each other.
One way to avoid this differentiation in print is to convert an RGB file to CMYK before printing. Another way is to use a Pantone Process Book to see how a colour will print in the final output.
It is important that the file for four color printing uses the CMYK colour mode.
In other prints, directly, no colour mode or ink is used, such as 3D metal printing.
What are the advantages and disadvantages of four color printing?
Four color printing is one of the main printing methods in the industry. As with everything, as well as having its advantages, it also has some disadvantages to be aware of, to avoid surprises
Advantages of four color printing
- Practically any colour can be reproduced from only 4 inks
- High quality printing
- More economical than other printing types
Disadvantages of four color printing
- Cannot obtain certain types of metallic and Pantone colours that cannot be achieved by four color printing.
- Colour variations may occur between printing and printing due to different factors.
What kind of products is four color printing used for?
Four color printing is used for almost all printing products that most businesses and individuals use on a daily basis:
- Flyers, brochures, leaflets, leaflets, leaflets, etc
- Business cards
- Catalogues, magazines and books
- Letters, posters, banners.
Practically the vast majority of photographic or digital printing work is carried out in four color printing.
Frequently asked questions on four color printing
Can any colour be achieved in four color process printing
No. Sometimes there are certain colours that cannot be reproduced exactly with the four-colour process. In this case, PMS colours (also known as spot colours or Pantone system colours) are used to create a particular colour.
PMS colours are specific colour formulas that are accurately reproduced in print. Instead of simulating colours by overlaying multiple ink colours with the CMYK 4-colour process, PMS ink colours are pre-mixed from existing colour formulas and assigned a standardised number.
PMS colours are often used in conjunction with the four CMYK process colours in certain projects. They are called 5- or 6-colour projects because they use the four CMYK colours plus one or two PMS colours (or more) for certain design elements, such as the corporate logo.
PMS colours usually carry a surcharge, so they are usually reserved for projects that require a specific colour (or colours) that cannot be accurately reproduced by overlaying the four CMYK colours.
Only a small percentage of full-colour projects require the addition of PMS colours because most graphic designers refer to a Pantone Process Book and then use the process colour formula that most closely approximates the desired PMS spot colour. Therefore, if you intend to print the entire piece using the CMYK 4-colour process, it is important that you do not use PMS Spot colours in your project design. Otherwise, when your PMS Spot colour is converted to a CMYK process colour to create the printed output, it may give a result that was not intended.
If in doubt, always consult with your printer before delving into your four color printing project.
Why are there colour variations in four color printing?
When you are printing a job there are several things to consider. The colour of your printed project will vary from printer to printer, from print to print and even between copies of the same print run.
Why does this variation in colour occur? There are several influencing factors, such as the percentages of ink used, environmental conditions, the printing press, etc. This results in small variations when printing in four-colour process.
It is very important to bear this in mind, so as not to raise false expectations.
What colour mode should a file be in for four color printing?
If you are going to do a four-colour print job, it is important that the colour mode of your print file is CMYK, because if you use a colour mode such as RGB, the colours of your printed job are likely to be very different from what you see on your computer screen.
To get the best possible result from your print, it is best to consult with a print design expert to review your file and confirm that everything is in the correct colour format.
Want to learn about other types of printing besides four color printing? Take a look at our printing types section.