CMYK is not a colour

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Ken Jones runs Circular Software. He was Technical Production Manager for Penguin and Dorling Kindersley for several years and has since advised publishers such as Parragon, Nosy Crow, Walker Books and Quarto on how to get the best from their print workflow.

I spoke at a recent BookMachine Unplugged event to challenge some of the received wisdom that designers and production staff follow even though they probably no longer should… in this post I take on our use of CMYK images.

Colour accuracy can be an important factor to the success of illustrated books. Even at a subconscious level we make judgements based on the colour of the things that we see. I have heard stories of cook books being pulped for featuring inedible-looking food and yoga books that seemed to be exclusively modelled by cadavers.

Also, we do judge a book by its cover of course, so even for mono publishing the colours used on the cover usually play an important role in getting our attention and stimulating a response.

So, naturally, publishers want to see the colours they will be getting from their final print run before their books go to press.

The best way to know how something will look when printed is to print it. But to make plates and run a commercial press up to speed just to make a proof is so expensive it is just not feasible.

However, if a press can be set up in a consistent and repeatable way then, by accurately recording the colours we get from one print run, we can be confident of the colours we will get next time.

The challenge for the modern print supplier is to run their presses in a consistent and therefore predictable ‘printing condition’. A printing condition takes into account a multitude of variables which include paper colour, coating and absorbency, standard ink colours, densities and dot gains.

By using a print supplier who is in control of their printing conditions, we can get a predictable result.

A colour profile is just a big list of numbers

Typically, an illustrated book or cover is printed with CMYK inks (Cyan , Magenta, Yellow and BlacK) on a sheetfed offset press. There is an ISO on how to set up printing presses and international printing standards bodies have already done the hard work of setting up presses, accurately printing a wide range of specific colours on common paper stocks. I talked about this more in another post.

These colours from the press are then recorded and a big table of numbers is generated that simply states that when you print a particular amount of CMYK in a particular printing condition you get this exact colour. Our computers are great at number crunching and by referring to these tables (called ‘colour profiles’) we get to accurately control our colours in Photoshop, Illustrator, InDesign and beyond.

Colour profiles work in two directions so, as well as knowing what exact colour I will get when I print an amount of CMYK, alternatively if I know what colour I want to achieve, I can translate it into the correct amount of CMYK inks I will need for a particular printing condition.

CMYK is not a colour. It is only an amount of ink

There is not one type of CMYK, there are many.

The printing condition has a huge bearing on how those CMYK values will be translated into colour. Locking our images into CMYK too early is a bad decision.

Nobody will congratulate you if your CMYK values are unchanged but your colours are inconsistent and look wrong. Your numbers should change and must change to preserve your colours. Using colour profiles to make sense of the colours in our digital images and managing how the numbers change to maintain colours, wherever possible, is called ‘colour management’.

Only by using colour management and associating the CMYK values with a colour profile can we know what colour we can get. Only by using a printer that can print to standard and repeatable printing conditions can we be confident we will get the colour we want.

Stay in RGB

There is not one type of RGB, there are many.

Digital photography and scans of artwork are measurements of light using RGB (Red, Green and Blue) and should attempt to capture the actual colours of their subject. Professional photographers will take a shot of a colour chart and build a colour profile for each shoot.

The colours available in RGB don’t always make it to CMYK. Oranges and greens are especially lacking. But by using colour management we can predict and proof the effects of printing on our RGB images.

An ‘Epson proof’ is an affordable digital proof that very accurately shows how colours will appear when printed with CMYK. It is important to remember that the colours it shows are an exact replication of those that have come from a real printing press run to certain printing conditions. With a good quality, well calibrated screen the effects of a CMYK printing condition can also be shown back to us on screen. It takes some effort to do, but it does work.

Changing an RGB image to CMYK is very easy to do in Photoshop but for regular photographic images there is no need. At best you are just wasting your time, at worst you are unnecessarily discarding colours, increasing file sizes and locking in the wrong CMYK profile.

If you are on top of your colour management then you don’t need to convert to CMYK by hand. And if you are not on top of your colour management then you really shouldn’t be doing it either!

RGB images are more flexible, hold a greater range of colours at a smaller file size. Regular photographic images should be converted to an appropriate CMYK colour profile automatically on output to PDF using PDF/X.