The Problem With Digital Colour
The problems with digital colour are caused by the differences in the way colour representation is handled by different devices. A scanner operates in the RGB colour model and only recognizes a limited number of distinct colours. A monitor screen operates in RGB and recognizes a different range of distinct colours. And a high-end photo printer such as the HP DesignJet Z-3100 or Epson Stylus Pro 9800 can operate in both RGB and CMYK colour spaces and recognize again a whole different range of colours, but will internally use the CMYK model.
The most urgent problems with digital colour occur when printing is involved. As long as the colour model is unchanged, colour management is more or less easy to understand. When printing is involved, the colour model may change from RGB to CMYK, and different colour spaces may be introduced, and a whole bunch of challenges demand the user's attention.
Design and photos that are created on a Mac or PC, with a scanner or a digital SLR camera, and that will be used for display on a monitor screen only, all stay within a RGB colour space. The working space may differ (we'll come to that later), but at least the colour model remains unchanged. Anything that goes to a paper output may drastically change the level of knowledge required from the user. For most commercial photographers and designers, the intricacies of changing colour models and working spaces should become a major factor of attention.
Colour Models
Photographers usually need to match input content from a digital camera or scanner to the display of their images on a monitor screen and output to an inkjet printer or dedicated photo printer. More often than not, their colour worklfow is all RGB.
However, commercial photographers will rarely deal with RGB alone. Most magazines are still printed and commercial photographs end up on printed pages as well as on the web. In a cross-media publishing world, commercial photographers need to understand and familiarise themselves with the colour space used by printing presses, i.e. CMYK.
Graphic designers --the people who deal with brochures and annual reports-- often work in CMYK only. Adobe Illustrator enables graphic designers to frget about RGB; they can work in CMYK, whereby they should keep in mind that what they see on-screen is a RGB representation of CMYK art. In a cross-media publishing world, they will often be required to output their work to the web. In Illustrator CS3, this is a matter of selecting the Export option for Web.
Layout designers are a hybrid. They have the most difficult job besides the commercial photographer. They should be capable of handling RGB images and CMYK images. They should understand how an RGB image will be converted into a CMYK image and what the consequences of such conversion are for the colour representation. In a well-organised workflow, layout designers should be told how to handle photographs (perhaps even refuse RGB photos from a photographer, and insisting on CMYK).
Up to a large degree, the layout designer's problems can be taken care of by a system like QuarkXPress 7's Job Jackets technology. The workflow can be set up in such a way that images cannot be placed unless they are CMYK. But even with an enforced CMYK workflow, the layout designer may have its colour profiles all wrong, or he may be using the wrong working space. If he and the photographer are not using the same working space, problems may occur.
How colour devices work
A monitor uses red, green, and blue (RGB) light to create colours. Combining the full intensities of all three colours makes white. RGB colours are used for lighting, video, and monitors. Your computer monitor creates colour by emitting light through red, green, and blue phosphors. The so-called RGB model is familiar to photographers, as it describes the way light creates colour. We call it an additive colour model because the different wavelengths of the spectrum are added together to create pure white.
A printing press and most inkjet printers use a CMYK colour model, in which three colours of transparent ink (cyan, magenta, and yellow) are combined along with black (noted as K, derived from "key colour") in varying amounts to create different colours. CMYK inks filter the light that reflects back from the paper and subtract some of the red, green, and blue light from the spectrum. White results from subtracting Cyan, Magenta, Yellow (and black) from the inks that are supposed to be pasted onto the page. In the subtractive CMYK colour model, white is the absence of ink on the page (provided you do use white paper). In theory, pure cyan, magenta, and yellow pigments should combine to absorb all colour and produce black.
But because all printing inks contain impurities, these three inks actually produce a muddy brown and must be combined with black ink to produce a true black. Any colour on the page that you see is what's left when a specific amount of one of the three primary colours CMY gets subtracted from a full coverage of CMY combined. Although your inkjet printer usually works through an RGB printer driver, the printer itself is almost always a CMYK printer (often 'enriched' with other colours to improve on colour saturation).
The difference between how your monitor interprets colour and how your printer creates colour is irreconcilable without some 'translation' between the two models. Simply put, we call the system of translation between the colour models: 'colour management'.