CMYK (for Those Who Do RGB)

February 11, 2005 11AM PST

Whether you just quit your job to pursue your dreams, or you’ve simply had a request from a long-time client, sooner or later you’re going to have to design a set of business cards or letterheads or something else that ultimately forces a trip to the printing press.

Since web geeks think in pixels and RGB, it’s a daunting new world to head into unequipped. Over the years across quite a few print jobs by now, I’ve had to learn by trial and error. As someone used to thinking in RGB, I’ve looked high and low for a good resource to turn to for help in converting that knowledge to CMYK. This may not be that resource, but I figure I’ve amassed at least the beginnings of a how-to on the subject. Consider it non-authoritative, but hopefully useful. Most of the knowledge contained within applies to Photoshop, mainly because I’ve had the most trouble with it.

CMYK vs RGB

CMYK Colour Diagram

First, the basics. For anyone who hasn’t owned a printer in the past decade, CMYK, or Cyan Magenta Yellow Black, refers to the colour of inks used in the printing process. CMYK inks combine in proportions to form solid colours; Yellow and Magenta with a bit of Black form a dark maroon, for example. But there are some major differences between the RGB colour model your computer display uses and the CMYK process many printing presses use, and that’s where the first point of confusion often comes in.

Rule Number One, above all else in print, is that what you see on paper will not look like what you have on screen. With tweaking it may be possible to bring the two closer in line, but suffice it to say that when first printing out your work there’s a very good chance you’ll find yourself horrified at the outcome. Never trust what you see on screen.

CMYK isn’t the final word in ink choice, it just happens to be fairly common. For business cards in particular, another potential choice is spot colour. No one says you have to print full colour, and thus use four inks. Effective business cards often use two or three, and the reason is simple: it can be quite a bit cheaper. But pulling out any of the three from the CMYK combination will produce ugly results, so spot makes use of solid coloured inks that span a wide range outside of what your standard CMYK combination is capable of. And specialized spot colours like metallic and neon shades are possible as well.

Of course, you can use more than four spot colours, and potentially combine spot and CMYK in the same printing, but once you start ranging into complicated print jobs the labour involved in producing them — and thus your final cost — quickly increases.

To further complicate things, CMYK can be printed digitally or on an offset press. The latter is a traditional printing press that requires a plate to be created for each colour of ink that prints. For CMYK, that means each of the four inks must be separated into its own plate and printed individually. As you might imagine, printing this way is more expensive due to the labour involved in creating the four plates, or separations as they’re known. A digital press, on the other hand, is more or less simply a colour laser printer. It may be a touch more expensive than your garden-variety office colour laser, but the output is pretty much the same. Printing digitally is generally cheaper.

Colour Management

Alright, next up — once you’ve chosen your inks, how in the world do you work with the colours on an RGB screen? This is where I’ve been stuck for years, and I’m still not sure I have a solid handle on it, but it’s worth passing on what I know (and following the comments on this article for advice from those who know better).

Note: if you haven’t mastered the curves, levels, and channels of Photoshop, now’s the time to brush up because you’re going to need them (the linked articles are the first respective results I could find on Google; they barely scratch the surface, you’ll want to keep digging).

First of all, we’ve established that an on-screen colour will not look the same printed. Again, there are major differences between RGB and CMYK, or more specifically, light-based colour and ink-based colour, that prevent this from ever being possible.

Pantone Swatches

Colour management is a deep and complex science, and the best coping strategy I’ve found so far is to try and work around it. Pantone, which you have probably heard of, is one of a number of colour-matching systems that basically gives you a palette of numbered, pre-printed swatches to choose from that accurately represent the final colour.

The advantage is that if you use that exact Pantone number, on that exact paper, you know exactly what you’re going to get. It won’t look the same on-screen of course, and your software has to support it (in Photoshop for example, when you have the colour picker dialogue open, hit the ‘Custom’ button to get a list of different colour-matching systems and their various palettes).

The disadvantage is that you won’t likely have your own set of printed swatches. An official set is pricey — a few hundred local currency for what is essentially a chunk of ink samples. You can pick up cheaper books off the shelf of your local art book store that are more approximations than accurate matches. Ideally though, your print shop will have a set on hand. If you’re able to make a trip first to go pick your palette ahead of time, you can save a lot of effort later.

Working With Process/CMYK

Colour matching systems work best in conjunction with flat colour and pre-defined gradients, so the text and vector output from Illustrator or InDesign, for example, is a great use case. But what if you’re working with heavily photographic material and don’t have that luxury? Start throwing in overlays and other forms of colour variance, and you’re going to quickly run into problems that Pantone won’t help. Without having direct access to the final output device, you’ll want to minimize test runs at the print shop to keep costs down (and avoid going back and forth to proof them) so you need to know ahead of time what you’re up against.

If you’re printing digitally, fortunately proofing is relatively simple — you have a colour printer, right? Inkjet or otherwise, these days you don’t even need a high-quality printer. A $50 special at the local hardware store is more than enough. The trick is that you’re not looking for high-end, ultra-accurate results; you’re looking for a rough approximation to see how much the colour is going to shift on you.

What I do is build my file in RGB from start to finish, and just before I’m about to print, I save a copy of the file and convert it to CMYK, then print that. If it’s a first shot, the proof will come out shockingly ugly and muddy, which means I’m on the right track. Again, this is just a rough approximation of the final colour, but it will give you a good idea of how far off the mark you are. Alternatively, Photoshop has a ‘Proof Colors’ setting (Ctrl + Y or Cmd + Y) which can apply the current CMYK profile and soft proof what it will look like on paper, but it generally won’t shift as much as an actual print will.

Now this is where the fun starts. How do you take that horrible output and get it to match the prefect vision you have in mind (and just finished creating on-screen)? Adjustments, and lots of them. You can either go back to the RGB original and adjust elements there, knowing what you now know about the final output, or you can work directly on a CMYK copy.

Adjusting Levels in Photoshop

What do you adjust? It depends on the output. If your blues are blowing out to a bright ugly green, it might be wise to back off the yellow and boost the black. If your reds are too grey, it’s worth toning down the black and bump up the magenta. How do you adjust? Again, this is where channels come in handy. I’ve had luck going back to the RGB file and lightening everything globally to tone down the black. For major 4-colour corrections it’s often worth it to flatten and save a copy of the file, converted to CMYK (Image -> Mode -> CMYK Color), and then apply curves or levels to each of the individual ink channels. I prefer levels for most operations, but more print-aware people will probably steer you to curves. Some selective masking to adjust the more problematic areas is often required as well.

Example Test Strips

Placing some superfluous bars of colour in the margins before you print can help cut down on the colour checks. If you know ahead of time which colours will be problematic, you can provide a range of tonal variations, mixing in varying proportions of ink. After a test run, assuming you’re in the right ballpark, you simply choose a better colour from the test strip and be done with it. Doing this cuts down on the printer runs trying to nail down the precise shade.

At some point though, you’ll have to get a proof from the print shop, to see how far off you were and to have a reference for further adjustment, if necessary. Printing digitally, this isn’t much of a problem — they don’t need to do much more than drop your file into their template and send it through the printer. Expect to have to come back later to view the result though, since they won’t drop everything to get your proof done.

Printing offset on the other hand, is a bit different. Unless you have the ability to create separations yourself, which you probably shouldn’t be doing anyway, the print shop will need to spend a bit more energy outputting those and running a proof. As far as I’m aware, the machine needs to be fully inked to print them too, so at this point the proof is what you’re going to get, barring any unforeseen disasters. It’s costlier to back out and make corrections at this point, although still possible.

Now, assuming your inkjet printer proved up to the task and the trial press run isn’t too far off what you were expecting, you should be ready to give the go-ahead at this stage. If you’re still not satisfied, you’ll have a guide to compare against the proofs you created earlier. You’ll be able to pick up spots that have changed dramatically between the two proofs, and assuming you can tell why they’ve shifted (basic colour theory applies here — yellow and cyan make green, red and yellow make orange, anything and black makes dingy brown or purple) you can adjust to suit. There’s a certain amount of guesswork still, but armed with your set of proofs you have an effective tool for comparison, which you can use to squash all but the subtlest of colour shifts.

Working With Spot

Let’s say you’ve decided to use spot colour, you’ve picked your Pantones, and now you’re ready to design. What next? Now you have to start thinking in limited colours, and your software is going to play a big role; nothing you know about CMYK translates to spot. My experience is limited to Photoshop and Illustrator, but there’s a relatively easy method of pulling off spot in each.

In Illustrator, create new swatches for your custom colours by clicking on the arrow in the swatch palette and hitting ‘New Swatch’. In the options menu, choose ‘Spot’ as your colour type and adjust the sliders to approximate your colour choice. Repeat for each ink, and you now have a set of spot colours to work with. Use them as solids, adjust their opacity, or mix them together in gradients; as long as you stick to this set, your Illustrator artwork will be set up properly for the print job. Technically they don’t even have to be the same colour as your final output either. Because each separation is printed as a greyscale version of the colour channel, it’s unaware of any actual hue; the Pantone you choose is what actually gets printed, not the on-screen preview.

Working with spot in Photoshop is a bit less straightforward. Essentially, you need to think of each ink channel as its own independent mask, and consider the limitations that come with that. Layers are out, for example, which also makes text uneditable; building a file this way is tedious at best.

Example spot channels in palette.

First, to understand how Photoshop works with spot. Open a photograph and convert it to CMYK (Image -> Mode -> CMYK Color), then go into the channels palette and drag one of the channels to the trash can at the bottom of the palette. Instant spot! You can view each individual channel in greyscale by making sure it’s the only one with the little visible eye icon next to it, or view them all as a blended preview of what they look like combined by enabling all eye icons.

Given your subject matter though, it’s probably none too attractive at this point. You’ll want to adjust the ink colours, which you can do by double-clicking on any channel (the ‘Solidity’ option is for your monitor only, it won’t affect the printed output). If you create a new channel while working in spot mode, it defaults to an Alpha channel, which you can then change to a spot channel by again double clicking, and selecting the appropriate radio button. Or just hit the arrow at the top of the Channels palette and create a ‘New Spot Channel’.

Working in spot mode.

Now what you’ll want to do is devise a strategy to build up each of your ink channels. Rather than working on a composite whole and selecting colours for each new item, you mix your colours by adjusting the ink proportions in various channels. Darker means more ink, lighter means less.

It’s a completely different way of interacting with Photoshop. Aside from layers, many of the usual tools are available to you when working in spot mode, and if you’ve spent time developing advanced masking skills you can adapt that knowledge and feel right at home. Otherwise, the least elegant way to work — but perhaps easiest to manipulate — is to build a set of greyscale files, one for each ink channel. Layers and editable text are usuable, but any areas that require more than one colour would need to be duplicated across both files, which makes editing tedious. You lose the ability to preview the combined channels though; the only way to get that back is to save a flattened copy of each greyscale file, then combine each in a new spot file, one channel at a time. Tedious!

Proofing spot colour is actually fairly simple; you know which Pantone colours you’ve chosen, so you can roughly match those in Illustrator’s colour picker. Make sure to print a test strip of your swatch to make sure you’re getting an accurate picture of the shift between the solid colours on screen and those on paper. Proofing in Photoshop is usually slightly more difficult due to the relative complexity of the subject matter compared to what you might be proofing in Illustrator, but the same process applies — make sure to print test swatches, keep your Pantones firmly in mind, and all but the most subtle of blends should be predictable.

Monitor Profiles

Finally, I’d be remiss to point out that there are advanced colour matching systems (both hardware and software) which have been developed specifically for the purpose of colour matching between screen and printer. Should you care? Probably not — they may save time for those spending a large part of their day thinking in CMYK, but for simple jobs where the basic concept of working with CMYK causes enough doubt to spur this article, it’s likely they’re overkill.

One thing that might be relevant though is the Monitor Profile your software uses. For general RGB work it’s best to turn off colour management; for printed work, the software needs to somehow interpret CMYK on-screen. There are colour profiles for different types of paper which depend on your geographical region; North America uses SWOP, for example.

Aside from accuracy in CMYK mode, the most useful function of having an accurate CMYK profile is the out-of-gamut warning you can pull up in Photoshop when working in RGB mode. By hitting Ctrl + Shift + Y or Cmd + Shift + Y to toggle on and off the warning, you get a layer of grey indicating spots in the file that are out of the CMYK colour range and most likely to shift dramatically. Simply by knowing these areas in advance, you have a map of which areas in your image need the most coddling, and much of your colour management issues obviate.