In today’s part of our “How to” feature, I’m taking a look at the differences between colour models and modes. The idea is to explain to you how another central principle of producing print files works.
Choosing the right colour space is crucial to offset printing, and so in what follows, I’m going to give you some information that will help you to avoid unforeseen colour discrepancies on your printed products.
About RGB and CMYK colour models generally
The very first thing to mention is that there are two relevant colour modes: on the one hand, there is the RGB mode (red, green, blue), and on the other, the CMYK mode (cyan, magenta, yellow, key). The latter is the colour space in which your products will actually be printed – with offset printing, a plate is produced for each of the four colours using CTP (computer-to-plate), and each of these thin, usually aluminium plates, is then locked into the right printing cylinder and as the paper passes through the press, each of the colours required is overlaid onto the sheet. This means that each printed piece consists of four layers, and the end image is produced by what is known as subtractive mixing.
CMYK colour model (subtractive colour mixing)
In theory, these three colours mixed together would be black. In order to get real black in printing, however, K (key) is used for details and contrast.
RGB is less about “real” colour than about the human eye’s perception of colour – e.g. our way of seeing colour on a computer or TV screen. In this model, the three primary colours red, green, and blue are mixed by light, which is called “additive” mixing. The overlap of the three colours leads to a range of different impressions of colour, and if red, green, and blue are added at the same level of brightness, the result is white; if green is missing, for example, then red and blue combine to make magenta.
RGB colour model (additive mixing)
This model is used to produce colour on monitors. If red, blue, and green pixels are all 100% bright, the result is a white screen.
Colour modes in practice
Most image sources (digital cameras, databases, etc.) offer imaging data in RGB mode, which makes it important to convert this data into CMYK mode for offset printing to avoid changes to colours. Given that the CMYK colour space is smaller than the RGB space, however, the offset printing process cannot reproduce all of the colours available on the monitor. The normal way around this is to produce ICC profiles for monitors, printers, scanners, etc., in order to try and offer a standard colour display on all input and output devices; professional graphics programs like InDesign or Photoshop also allow you to simulate the result with a good degree of accuracy. Then again, you need a monitor which can be calibrated, and you have to choose CMYK to print the product in any case
If you want to print areas, images, or even complete flyers in black and white, you’ll need to remember to save this files either as greyscale or in CMYK mode using the black channel only because if these tones are made up of cyan, magenta, and yellow, offset printing will give you dark brown or blue tones rather than proper black.
Spot colours
If you are preparing letters, business cards, or any other kind of business materials for yourselves or for your customers, then you will probably be required to keep to a corporate design of some kind. In most cases, specific colours needs to be used for things like logos or backgrounds, and they are supposed to look as similar possible on all media. If that is the case, you should definitely think about printing with spot colours, e.g. using the HKS or Pantone classifications.
HKS, which is very widespread here in Germany, is named using the initials of the three companies which came up with the system: Hostmann-Steinberg, Kast + Ehinger und H. Schmincke & Co.
A spot colour is a pre-defined colour tone which is given a unique name or number in the system catalogue, and using these spot colours allows you to print a tone outside of the possibilities of the four-colour printing system. This allows you to print highly-saturated colours or effect tones on a full range of materials.
HKS colour chart | Anja / pixelio.de
Pantone colour chart | Anita Winkler / pixelio.de
When using spot colours, you should make sure that no other colours from the CMYK mode are overlaid – i.e. make sure that the overlay function is deactivated, otherwise your object could be hidden or the colour’s appearance modified. If you use spot colours in a print file without deactivating overlay, the finished piece could also have areas that are untidy or spotty, so try to use trapping rather than overprinting processes. With this level of complexity, when using spot colours, it really is essential to look at the print preview in your prepress file to make sure that there are no serious errors.
