Most of you are no doubt familiar with the abbreviation CMYK already, but do you know what it is that makes this colour model so important within the printing industry? In this article, I hope to shed some light on the CMYK colour model for you by outlining the key things you need to know about this subtractive colour model.
CMYK is what is known as a subtractive colour mixing model. The basic colours used are cyan (C), magenta (M) and yellow (Y). In multi-colour printing, black/key (K) is added alongside these three chromatic colours to improve the contrast.
During offset printing, all the colours being represented get transferred to the paper. Layers of coloured ink are placed one on top of the other to alter the light spectrum of the (white) printing substrate. In other words, less light is reflected/allowed through, which reduces the light spectrum (subtractive = reductive). The possible values for each of the four individual colours range between 0% and 100%. In each case, 0% equals no colour and 100% corresponds to full colour.
Within the printing industry, CMYK colour printing is called “Euroscale printing” and the term “Euroscale” is used informally (particularly in the USA) to mean European offset printing. However, the technically correct name for Euroscale is actually the standardised “ISO scale”. So that this standard can be applied appropriately in specific situations, we use what are known as ICC profiles, one example of which is the “ISO Coated sb” profile for printing on coated paper.
The use of four basic colours results in colour space “overdetermination” and so the basic colour of black must be defined for each CMYK colour space. The sharpest definition can be achieved by ensuring that none of the secondary colours involve combining more than three basic colours. This means that each secondary colour must either be defined using three chromatic colours (chromatic composition) or two chromatic colours plus black (achromatic composition). Other key colorimetric properties apply, depending on the separation method used, the paper and the printing conditions. There are probably at least as many CMYK colour spaces as there are potential combinations of the factors referred to above.
Compared to the RGB colour space, the CMYK space is relatively small, i.e. far fewer colours can be represented with this method than they can on a screen. Tones within the turquoise or orange ranges are particularly problematic. For a comparison of the CMYK and RGB colour spaces, see this blog entry.
To get a slightly clearer idea of the difference between the CMYK and RGB colour spaces, you can refer to the following CIE standard colour table. This CIE standard system was defined by the International Commission on Illumination (CIE – Commission internationale de l’éclairage) to establish a relationship between human colour perception (colour) and the physical colour stimulus involved (colour stimulus specification). It covers the entire range of perceptible colours. By referring to the CIE standard colour table, you can get some idea of how the RGB and CMYK models differ in terms of their scope.
To ensure high-quality printing results, the total for the individual colours must not exceed a specific maximum value. In this context, the maximum value depends on the paper, which can only take a certain amount of ink. Colour distortions can occur due to what is known as the “blotting paper effect”, which is when different types of paper absorb the ink to varying degrees even though the mixing ratios are exactly the same. For example, in the case of the coated paper that is frequently used in offset printing, a maximum limit value of 300% is recommended.
To avoid disappointing printing results, it is advisable to use colour calculators that convert RGB colours into CMYK colour mode. In the first instance, I would recommend using the onboard tools in Photoshop or Illustrator. Alternatively, there are lots of online and offline tools out there, some of which are available as freeware. One example is the Color Conversion tool.
You may also come across the term CMYKT. The T stands for topcoat, which is a special finish that is applied to the print to improve its properties in terms of light resistance, sheen and water repellency.