CMYK Color Model explained

Printing colours has been possible since Gutenberg’s time, but the process of re­pro­du­cing coloured images for printed mass media, for example, was realised a few hundred years later. Stand­ard­ised colour scales have been around since 1915 with the Standard Colour Reference of America. Today, many homes will own a colour printer. But how does an RGB image get from the computer screen onto the printed paper? And why does it take four colours? We’ll show you what you need to know about CMYK colours and the dif­fer­ences to RGB and other colour models.

The use of colours for present­ing on screen and in print follows the physical char­ac­ter­ist­ics of the human eye. The eye is par­tic­u­larly sensitive to three light wavelengths. They are red, green, and blue and are used as part of the RGB colour space. This ensures a colour-balanced re­pro­duc­tion on colour monitors.

During the printing process, however, white paper is usually printed on. Paper is not trans­par­ent and therefore doesn’t allow light to pass through. That’s why the four printing colours cyan, magenta, yellow, and black - CMYK - are applied one after the other in in­dus­tri­al offset colour printing. The actual printing process is based on placing small, closely spaced dots which the human eye perceives as a colour image once combined and viewed from a distance. To produce an adequate image quality, in­dus­tri­al offset printing usually requires 300 dots per inch (dpi). The tiny dots are of different sizes depending on the desired thickness of the colour ap­plic­a­tion. If a colour component equals zero percent, there is no dot at this point and therefore no colour.

Similarly, with colour ink printers, tiny drops of ink are placed onto the paper in the shape of closely spaced dots. They range from 0.3 to 0.4 mil­li­metres in size. It’s the sum of the in­di­vidu­al droplets in the CMYK colours that creates the ‘illusion’ of a coloured image. A colour laser printer delivers the same result, but in a slightly different way. There are four elec­tric­ally charged drums, which are dis­charged by a laser beam where no toner should be released from one of the four CMYK cart­ridges. The remaining charged areas are then thinly coated with the colours. These coated print pixels are suc­cess­ively trans­ferred to the paper and fixed by applying heat.

The three examples – offset, colour ink and colour laser printing – il­lus­trate the demands on the precision of the printers. In the following, you’ll find out how colours are divided and used for this process.

The CMYK colour model is used when a rep­res­ent­a­tion of an image is phys­ic­ally output in the form of print media – i.e., using print machines of various tech­no­lo­gies, and computer laser or inkjet colour printers. In a four-colour laser printer, three more ink cart­ridges are used in addition to black toner – cyan, magenta, and yellow. The same applies to colour ink printers, although ad­di­tion­al colours have been used for some time now. Given the higher demand on printed products, in­dus­tri­al printing machines have also been sup­ple­men­ted with ad­di­tion­al colour printing units. In six-colour printing, they are green and orange. Other mul­ti­col­our printing tech­niques use defined spot colours.

RGB is always involved when something is displayed on screens or monitors. Digital cameras also produce RGB image files. With RGB, 16.8 million different colour tones can be achieved.

Every printable medium, including every type of paper, re­pro­duces colours slightly dif­fer­ently despite using the same CMYK values. Some absorb the ink more strongly, while others have a smooth, glossy surface, and others are matt. To com­pensate for this, stand­ard­ised colour profiles were created to ensure the same printing results are achieved for various types of paper and ink cart­ridges. An example of a colour profile like this is the stand­ard­ised colour profile ‘ICC V4’.

Colour profiles are available for scanners and monitors too to match colours across different technical devices for maximum colour fidelity across different output media. The data sets for stand­ard­ising colour man­age­ment systems are co­ordin­ated by the In­ter­na­tion­al Colour Con­sor­ti­um (ICC).

Colour systems expand on this and define the com­pos­i­tion of colours in greater depth. Some rely on the­or­et­ic­al models of colour per­cep­tion of the human eye.

The colour system is also referred to as CIE-L*a*b* or Lab colours. It is based on the human per­cep­tion of the so-called normal observer and includes all per­cept­ible colours. It is not based on a math­em­at­ic­al model like RGB or CMYK. The un­der­ly­ing standard is EN ISO 11664-4 ‘Col­our­i­metry – Part 4: CIE 1976 L *a*b* Colour space’. This is often referred to as ‘tri­s­tim­u­lus’ in spe­cial­ist articles.

The HKS colour system currently consists of 88 basic colours and 3,520 full tone colours that can be used to print on natural and art paper. In contrast to CMYK, the colours are already pre-mixed before the printing process. The aim is an objective com­par­ab­il­ity and re­pro­du­cib­il­ity of various colour nuances. HKS colours can be simulated using CMYK, but a similar bril­liance is rarely achieved.

The Pantone Matching System is a colour system that’s used in­ter­na­tion­ally for graphics and print media. It was developed by US company Pantone and contains 1,867 spe­cial­ist colours (as of 2016) that cannot be displayed using con­ven­tion­al four-colour printing.

The RAL Classics colour system from 1927 comprises 213 colours. The RAL design system with 1,625 stand­ard­ised colours has existed since 1993. Some RAL colours are corporate colours or signal colours for certain items, e.g., b. RAL 3024 is used to paint fire and rescue vehicles, rescue cruisers, and lifeboats. The true-to-colour rep­res­ent­a­tion of RAL colours is not uniformly possible on monitors and printers and is only ap­prox­im­ated when using CMYK or RGB colour spe­cific­a­tions.

Find out more about the use of colours and image formats in our guides ‘What is a pixel’, ‘Ten image formats in overview’ and ‘What’s TIFF? An in­tro­duc­tion’.