RGB / CMYK Conversion

Commercial printers do most of their full-colour halftone work using just four ink colours. These are cyan, magenta, yellow and black, abbreviated to CMYK where the K stands for Key, to avoid confusing it with B for Blue.

The reasons for using CMYK start with the way the human eye detects colour in the first place. Human eyes are sensitive to three overlapping wavelengths within the visible light spectrum, whose peaks are perceived by the brain as red, green and blue light, abbreviated as RGB. This is a big oversimplification, but it works for daily understanding.

Colour film, scanners and digital cameras are also sensitive to RGB, while TV screens, computer monitors and film exposure units that glow with their own light, generally create colour by mixes of RGB.

RGB colours are called additive colours, because the cumulative effect of two or three coloured lights can create new colours as well as a brighter image. For instance, red plus blue light makes purple/violet/magenta, red plus green makes yellow/orange/brown, and blue plus green makes cyan/turquoise. Equal mixes of all three produce white and shades of grey, while an absence of light produces black.

Unfortunately if you print in mixtures of red green and blue ink you get an awful mess. Paper does not glow and must rely on reflecting light from its environment.

For full-colour printing you use a white paper overlaid with transparent inks that act as selective filters for the red, green and blue wavelengths of white light. These are called ‘subtractive colours’, because they absorb wavelengths of white light as it passes through the ink, bounces off the paper and passes back through the ink again towards the eye of the reader.

So cyan ink ‘subtracts’ red wavelengths from white light, leaving green and blue to reflect off the paper. Magenta subtracts green, to leave red and blue. Yellow subtracts blue, leaving green and red. Two or more inks subtract more wavelengths, so the resulting colour is darker.

In the 'conventional' print processes of lithography, flexography, gravure and screen, the cyan, magenta and yellow inks are never mathematically pure shades, meaning that some colour management adjustment is needed to reproduce the intended colours. It also means that full-strength mixes of cyan, magenta and yellow can never give a solid black, but a dark brown. Consequently a true black ink is also needed, completing the four-colour ‘process’ set.

Using a true black also has other benefits. For instance it can be used to print small text (which is usually black) in a single colour so it is sharp – making it from three colours would inevitably give mis-register. Black is also used to create darker shades of colour in a photograph, rather than by adding heavier weights of the other inks which would take longer to dry.

Even the best CMYK printing can only reproduce a fraction of the ‘gamut’ (range) of colours detectable by the human eye, or by colour film and digital cameras. CMYK cannot accurately print bright oranges, blues and violets, emerald greens or even optically pure red.

The choice of four colours is actually a historic compromise between quality, expense and optical limitations. With the original regularly-spaced halftone dot screens, you must angle them differently to prevent the diamond-patterned optical interference effect called moiré. In the old days of cameras and film the limited range of possible angles meant a maximum of four colours.

Modern computer-generated irregular screening techniques are moiré-proof, allowing six and seven colour ink sets that reproduce a wider gamut. However, they need extra plates and ink so they still cost more to print. So extended-gamut sets tend to be confined to packaging and label printing where brand colours are important, and to fine art reproduction where low cost is less important than colour fidelity.

The latest digital print processes, particularly inkjets, tend to have purer CMYK ink colours than offset or flexographic press inks, so they can reproduce a wider gamut. This makes them particularly suitable for proofing, using colour management software such as GMG’s ColorProof, which in effect winds down the original colour purity to simulate the ‘dirtier’ colours of a press.