Plenty has been written about the holy grail of color consistency -- getting your screen and your print output to match across Photoshop, Illustrator, and InDesign (and Acrobat too). Adobe themselves will even talk about the newer setting in Bridge or in Adobe Creative Suite 2 in general which allows you to synchronize your color settings across the suite. And the nice thing is that it works -- for the most part.
But even with these settings, we still find those out there who complain of colors not matching. Mainly, these are files that include spot colors. I've seen some people talk about these issues, and I've seen a few suggestions for solving the issue as well. In fact, it always seems that spot colors throws a wrench into the workflows we depend on every day. A while back I discussed how spot colors and transparency can present some unexpected results (and I presented a solution to the issue).
I've touched on the real solution to this problem before, but only in smaller specific cases. Here, I hope to present a clear understanding of these settings that accomplish three very important things:
1. Your spot color plates are preserved.
2. The integrity of your spot color CMYK values are preserved should you choose to convert your spots to process late in the workflow.
3. Your onscreen proofs AND your composite proof prints that contain spot colors will match accurately across Illustrator, InDesign, Photoshop, and Acrobat.
I'll start by saying that some have offered solutions of switching the spot color definitions in Illustrator or InDesign to use LAB values (which match Photoshop) -- but this can be VERY dangerous and results in the loss of number 2 mentioned above. Sure, we all know that a designer should use spot and process colors as needed, but we also know that many designers will spec spot colors (either because it's easier to do so by picking them from a book, or because they don't know any better). In addition, printers may convert spots to process late in workflows as job specs change under deadline. Item 2 listed above can often mean the difference between a job that prints correctly or one that must be reprinted (and at who's cost?).
UNDERSTANDING BOOK COLOR
So first, let's understand something that was introduced to Adobe Creative Suite 2: Book Color. You may notice that at times, a swatch will show up in Illustrator CS2 as something called a "book color". This occurs in InDesign CS2 as well, and here in the United States, you'll find it most often when you specify Pantone colors. So what is this book color setting? Well, here's the story: one of the reasons why we use spot colors is simply because there are colors that simply aren't reproducable in CMYK. Bright blues, purples, greens and oranges are perfect examples. But you also know that there are CMYK conversions of these Pantone spot colors. Pantone delivers an entire Excel spreadsheet to Adobe that lists each Pantone color, along with their LAB color equivalents. Obviously, the LAB color values match closer to the real color. Pantone also delivers a Solid to Process library which specifies the CMYK values of Pantone colors. Obviously, the latter is far less accurate and results in color shifts (and if you're paying attention, you know that by default, Photoshop uses the LAB values and AI, ID, and Acrobat use the CMYK values, which means that placing PS content into the other apps can result in color shifts).
Traditionally, a single swatch can only contain a single color definition. Since Illustrator or InDesign always used the CMYK values, proofing spot colors from these apps -- either on screen or on a composite proof -- was less than desirable. Especially considering the statement made in the previous paragraph, color shifts are also common due to this.
A book color swatch is a swatch that contains BOTH the LAB and the CMYK values of a spot color. In this way, Adobe can be more intelligent and use the values that are best for the situation. But there's a catch. You need to specifically tell Illustrator (or InDesign or Acrobat) that you want to use one value or another for the task at hand. But the important thing here is that you don't want to choose one OVER the other -- you always want to have both at your disposal. By this I mean, when you are actually printing and separating as a spot color, you want the best possible proof. But when you decide late in your workflow to convert your spot to process, you don't care about the LAB value -- you care about the CMYK breakdown and the consistency of that breakdown with other artwork, apps, etc. If you instruct your app to use the LAB color space for your swatch, then if you ever convert your spot to process, you get a LAB to Process conversion -- NOT the CMYK breakdown that the spot color manufacturer has specified in their book.
So what do you do? You use a feature that you would normally never imagine would have anything at all to do with this.
OVERPRINT PREVIEW TO THE RESCUE
OverPrint Preview is a wonderful feature that exists in Illustrator, InDesign, and Acrobat. But it's the worst named feature ever to grace those programs. That's because you might think that OPP only simulates or displays any overprint commands on your screen (which is very helpful). But it also goes a step further and uses a different algorithm for displaying spot colors. When you have a book color specified, we discussed that the swatch contains both LAB and CMYK values. In normal Preview mode, AI, ID, and ACRO all use the CMYK values for displaying the colors on your screen. But when OPP is turned on, the apps display those colors on the screen using the LAB values in the swatch.
So the first thing we've learned is that if you want to see consistent spot colors on your screen across all of your applications, don't touch the swatches -- rather change your preview mode by turning Overprint Preview on. This way, the integrity of the swatch remains, but you're basically instructing your application to display the color on your screen using the more accurate and consistent LAB values.
But the question is, what about composite proofing? I understand that I can use OPP to SEE my colors in a consistent way on screen, but how can I also get color proofs without color shifts? The answer is...
I always wondered why if I create a file without any overprints in it, I can go to the Print dialog and see that the Simulate Overprint command is available. If there are no overprints present, the item should be grayed out, no? The answer is that when you turn on simulate overprint in the Print dialog, the same thing is happening as we discussed above with Overprint Preview. The printer is using the LAB values for the color, not the CMYK values.
So the second thing we've learned is, if you want to see consistent spot colors on your composite proof prints, turn Simulate Overprint on in the Print dialog box. Again, this feature is available in AI, ID, and ACRO.
If you use OPP and Simulate OP, then you never lose the CMYK definition of the color that was specified by the book manufacturer, so it's always safe to convert to process at any time in the workflow if necessary.
In closing, you can ask why AI or ID even allow users to switch their spot colors to use only the LAB equivalents at all, if based on what you read here, it's such a bad thing. The answer is that in a color managed workflow, printers or RIPs may be able to better match spot colors to specific press or paper stock conditions when going straight from LAB. So I would strongly suggest that if you do want to use the methods described by others (such as Steve Werner's suggestion which I've linked to above), you first speak with your printer -- or you make sure to never convert your spots to process.