Colour Spaces

"What colour space should I build my artwork in?". Mike Adams discusses colour spaces, specifically for a wide format workflow.

Mike Adams of www.correctcolor.org has kindly given us his permission to reproduce this article on Colour Spaces. It originally appeared in Wide Format Imaging in February 2008, and can be seen in its original form by clicking here.

"Well we've found that a good printer profile can cover for a whole host of sins." —Name withheld

Name withheld because that takes the prize as the dumbest statement I've heard to date in my travels as the Johnny Appleseed of wide-format colour management. It was made by a fairly well-known solvent machine dealer in a good-sized American city and it's tantamount to saying, "Well we've found that if you make a good penthouse, you can just forget all about that pesky foundation." 

If you want to get a good argument going amongst digital imaging types, one way to start is to bring up the whole idea of working colour spaces: Which ones to work in and why; whether to work in RGB or CMYK; when to move from what space to what. Either people have an opinion on what the best colour space to use is, or many have the opinion that it's just all completely too involved and intricate a subject for the poor unwashed masses in the industry to ever understand. (Which is decidedly untrue, by the way. I always set up working colour spaces for my clients and they never fail to grasp the concept. Usually what people who say such a thing mean is that they don't understand it themselves, and it's easier to say their clients are too stupid to get it than to try and learn it themselves.) 

Colour spaces are the foundation of digital imaging. We couldn't do what we do without them. 

Myself, I think of colour management in my own terms of the 'front end' and the 'back end.' Basically and hopefully easily put, the front end is everything that happens before you send your files to the RIP, and the back end is everything that happens after. And what the gentleman I quoted above missed is that if the front end isn't right, the back end is doomed from the outset.

 

In the arena of wide-format imaging, the front end can be broken down into the two components of what colour space to use, and how to make sure your colour is unchanged from application to application. Of course, right off the fur can fly over the whole issue of whether to work in CMYK, or in RGB. And equally just as soon as the fur starts flying, people can spout commonly assumed falsities to prove their points. One key to remember is that if anyone ever says CMYK or RGB, and they don't follow up with what CMYK or what RGB, there's a good bet they're going to take you down a wrong path. That's because RGB and CMYK are both what are known as "device-dependent colour spaces." The key, the absolutely fundamental, critical, write-this-down foundation of getting control of your colour workflow, is that you can't say RGB or CMYK alone. Those aren't good enough definitions. You have to pick and define and implement both your RGB and CMYK spaces. 

One of the problems in this industry is that a lot of terms have overlapping meanings. They overlap even when referring to the same files or the same processes. And therein certainly lies some of the confusion when talking about the whole issue of working with RGB or CMYK files. Fact is, that as used in the context of describing the makeup of a file, the terms RGB and CMYK only refer to the numerical definitions of each pixel in an image. Your screen--for instance--creates colours out of red, green, and blue. If you convert an RGB file to CMYK while you're viewing it, you didn't just make your screen into a CMYK device, you just altered that file's pixel information from—basically—three numbers per pixel (red value; green value; blue value) to four numbers per pixel (cyan value; magenta value; yellow value; black value.) 

That's it. The first thing I tell clients when I do a colour space demonstration is that computers are stupid. Fighting colour issues constantly again and again and again and again, it's easy to get the idea that computers are devilish and gremlin-filled and they cackle in sheer delight as they turn grey to green and purple to brown at the most inopportune of times, but that really isn't true. What's true is that computers don't know the first thing about colour. They only know numbers, and each and every time you tell them to do something, they'll do exactly what you say. If you pick a workflow colour space, tag all your files and conform all your applications to that space, then you'll get consistent colour each and every time. If you don't, if you leave all your applications to their own default devices, then each time you take a file through an application with a differing default colour space, you'll alter the colour information of the pixels in your file.

 

More often than not, that right there is the cause of folks issues with colour and colour management. However, what happens is that they then go off chasing entire flocks of wild geese to alter this profile and download that profile and try this or that at the printer or in the RIP, and that isn't even where the problem is. 

So how does it work? Pretty simply, actually. Every digital image is, of course, in the end just a bunch of little boxes with numbers in them. Pixels. The numbers—also of course—relate to colour. In 8-bit RGB, each pixel contains a number 0-255 for red value, a number 0-255 for green value, and a number 0-255 for blue value. And again, that's it. That's the end of the story and nothing more. 

The key is, 0-255 as relates to...what?

 If one RGB space is bigger than another RGB space, then, for instance, it might be able to get a much greener green than the other space. Problem is, in both spaces, green as green can be is going to be defined as 0 (no red) -255 (green as green can be) -0 (no blue.) Unless you tag all your files with the space in which you worked them in one application, and unless all your applications are set up to honour the tag and work in that colour space as well, what they'll do as you move your file between them is assume the pixels in your file are defined as whatever colour space is their default. And change your file for you accordingly.

 Helpful. Okay, so, all the more reason to work in CMYK then, right?

 Wrong.

CMYK is device dependent too. In a CMYK file, there are four numbers per pixel, related to you by all the applications for easy understanding as cyan, magenta, yellow and black values, 0-100 percent. That's why, incidentally, CMYK files are bigger then RGB files. They've got one more piece of data per pixel. But again, those values have to relate to something. And what they relate to in each case is a certain colour of ink printed at a certain density. In the case of the Adobe applications' default CMYK space of SWOP, the colours are litho process cyan, magenta, yellow and black printed on coated stock at SWOP densities. 

And the fact is, SWOP is a relatively tiny gamut compared to most wide-format imaging devices. But more to this point, it again is just one device-dependent colour space. Not all applications use it as a CMYK default; so once again, as you move from application to application, if you don't tag all your files and set all your colour space information in all your applications the same, each application will assume your file's pixels colour values were created using its own default colour space, and helpfully alter them for you accordingly. 

All of this is stuff that is happening before you ever even consider a printer profile. That's why all of this is the foundation of colour management. There are disagreements and arguments and theories about what colour spaces to use for working space and why—and I have my own very strongly held opinion there as well, which will most likely be another column—but whatever and however it's done, picking a colour space for your workflow, setting all your applications to that colour space, converting all incoming files to and creating all new files in it, is the absolute foundation of colour management. Don't do that, and there's no point in looking for salvation in a pretty penthouse. 

Mike Adams is the owner of Correct Colour, Austin TX, specializing in complete wide-format colour workflow management. He has been in the business of putting colorants onto media for parts of four decades, and was one of the pioneers of colour-managed wide-format workflows. Contact him at 817/781-4010, typhoon@correctcolor.org, or at www.correctcolor.org