RGB or CMYK? The CMYK habit discussed!

"Submit all Artwork in CMYK?" - nonsense! Does your wide format supplier request CMYK files? Ever wondered why? There really is no justification for the blanket application of a CMYK workflow in a wide format context. If you're needlessly converting your vibrant RGB photos and designs to CMYK before sending out to print you will be familiar with the deadening of colour this process can achieve!  What a way to treat a vibrant design! There are a few examples where converting all of your artwork to CMYK has a benefit, but in most instances, all it does is limit the colour available to you. In this piece, Craig Hudson explains why you should expect so much more of your wide format supplier.

OK, so I've been a bit melodramatic. There's nothing wrong with CMYK colour spaces. Some CMYK spaces are pretty large, and won't kill your colour. Also, if you're trying to match prints produced via other methods, there's a valid argument for using a smaller CMYK colour space. But most of the time you see "submit in CMYK" the space isn't being specified. What's really meant is "submit in the SWOP colour space". To work that way by default. What a sad waste. How many times have you looked at a printer's artwork requirements and they've started with the comment "please supply all artwork in CMYK"? Next time it happens, be sure to ask "which CMYK" and if they pause before answering, run for the hills. You see, CMYK numbers alone only tell part of the story and won't lead to accurate colour reproduction. You need to specify the colour space you're in, and your printer needs to understand how to use that information to output colour on their machines. Don't worry if that seems complicated right now, the concept is straightforward and you don't need to be a colour scientist to grasp it.

There are three separate discussions here: 1) Why CMYK (or RGB) numbers alone are not enough - understanding what colour spaces are. 2) Why assuming the SWOP colour space leads to the same boring colour everyone else has. 3) How you can make your colours POP with no extra effort!

1) Why CMYK (or RGB) numbers alone are not enough - understanding what colour spaces are.

Consider these patches (Figure 1) and look at the colour values within those patches.(you might need to use your browsers zoom feature "CTRL" & "+" on most browsers.) These are all identical as far as the CMYK numbers are concerned! Cyan 30%, Magenta 50%, and 0% Yellow or Black. If you only specify CMYK numbers a printer would be perfectly correct supplying you any one of these colours. Is that variation acceptable to you? It maybe that "in the right ballpark" is good enough for your needs. If not, you need to define your required colour more accurately in the first place. You do that by defining the colour space you are working in.

CMYK and RGB Colour Spaces are device dependant. That means, they refer to the range of colours a specific device is capable of producing. That range for colours is referred to as the "gamut" of the device in a given condition. For example, Figure 2 shows the gamut of Hudson's Mimaki JV3, printing with Mimaki's SS2 2xCMYK inks, on LG's LD3810 Gloss Vinyl.

Because every material and inkset is different, every product combination has its own unique gamut. There are also some special colour spaces - for example, US WebCoated SWOP v2. (Figure 3) This is the default CMYK colour space of Adobe CS. If you don't tell Adobe CS to use a different CMYK space, this is the CMYK space you'll be working in by default. How is US Web Coated SWOP v2 a Device Dependant colour space if every one has it by default...? Well SWOP (Specifications Web Offset Publications) represents a specific set of inks, at specific densities, onto a specific paper stock. The principle is that offset printers "tune" their presses to meet this standard, so that there is consistency of output across presses. 

The key thing to understand about any colour space, CMYK or RGB, is that when you use the numbers, either CMYK or RGB, in two different spaces, they represent two different colours. Without knowing the colour space, we don't know the colour the numbers are guiding us to. 

Now consider the two colour gamuts shown here in Figure 3 and Figure 3b.

Hudson's LD3810 gamut, and US Web Coated SWOP v2. If you specify the colour 30C/50M/0Y/0K in each you see two very different colours. To tell someone which one you mean, you have to specify which of the spaces you're working in. You do this by embedding the ICC profile, which tells the program opening the file, what colour space the file is in, and therefore what exact colour the CMYK numbers refer to.


2) Why assuming the SWOP colour space leads to the same boring colour everyone else has!

The "Only Send Artwork in CMYK" idea seems to stem from offset work, where presses are set up to output to the set standards (SWOP). Effectively all CMYK numbers are assumed to be in the SWOP colour space. Because Adobe still use this as their default colour space, Hudson's will assume SWOP as the colour space if nothing else is specified. So, what's wrong with that?

To reiterate, the solid shape in Figure 3 represents the SWOP colour space. This is all the colours you can specify within the SWOP space. Your 100% colour values represent extremities on this shape - you cannot specify colours outside of the space. This is all you have. That's fine if you're sending work to print on a web offset printer, because this is all the colours it has if it's set to the SWOP standard. So, the advantages of supplying your artwork in SWOP CMYK are that you'll get a consistent result across many sorts of media.  BUT there's a HUGE missed opportunity.

Wide Format printers aren't restricted to SWOP. Most wide format media and ink combinations can achieve a much larger gamut. In Figure 3 the solid shape represents SWOP, but the wireframe shape represents the colour gamut of Hudson's LD3810 vinyl on the Mimaki JV3 again (as seen in Figure 2). You can see that there are many colours the Mimaki can print on this vinyl that a SWOP restricted printer can't achieve. All of those colours in the space between the two gamuts can be printed on the Mimaki, but can't be asked for if you create SWOP artwork. What this means in a practical sense is that if you submit artwork in the SWOP CMYK colour space you are unnecessarily restricting the colours in your artwork. There's nothing wrong with that, you'll get an accurate print of your artwork, but by submitting artwork in SWOP you are effectively saying "ignore the bright punchy colours the inkjet machine is capable of or that were in my original design, I want you to limit the colours to those of a web offset printing press."


3) How can you make your colours pop?

In Figure 4 the same Mimaki 3810G gamut is the smaller shape, whilst the larger shape is the Adobe 1998 Gamut. To remind you, these shapes represent the range of colours you can describe in each colour space. Now the situation is different.

If you create artwork in AdobeRGB 1998 you can describe colours to (and beyond) the extremes of the printers capabilities. The only thing remaining is for us to determine what to do with the colours that you've specified that are outside the capabilities of the printer - and we're very comfortable with that process. (see Rendering Intents in our knowledge base.)


In Conclusion:

CMYK or RGB numbers do not tell anyone what colour is required without also knowing WHICH CMYK or RGB is being referred to. You can pass on this information by embedding the ICC profile within your file. You can also simply tell the person who has to open the file, and they can apply the correct profile at their computer.

Using the SWOP colour space is fine, but it fails to utilise the full colour possibilities of wide format inkjet devices. (Note: If you were sending the artwork to both Litho and Wide Format Digital Printers and you wanted a close match, it would make sense to use SWOP - thus limiting the wide format machine to the gamut of the other device.)

By using a colour space such as Adobe RGB 1998 you can use colours designers working or providing artwork in SWOP can't ask for. Your designs will shout where theirs merely whimper.    

When a wide format printer asks you to submit in CMYK (and especially if they don't say WHICH CMYK) they're basically saying either "we don't know how to handle colours that we can't print" OR they're saying "we're playing it safe; we want a safe easy life where any print we do will match items printed on smaller gamut machines. Our clients will live with safe subdued prints." Using SWOP correctly it is a valuable tool in gaining consistent colour across different print methods. Using it by accident or default is a travesty! The question is, are you happy to miss out on the bigger bang wide format printers can give you, or compromise your colour to accomodate a supplier's lack of knowledge or their desire of an easy life? If not, you do have a choice - find a supplier like Hudson's who welcome these discussions, and take your colour seriously!


Craig Hudson is the International PrePress Associations first qualified Colour Management Professional to achieve Master status, and the first UK based IPA qualified Colour Management Professional. Craig is Managing Director of Hudson Display Services Ltd, although he prefers the title "Chief Colour Wrangler" and may well have his business cards adjusted to match.