First, an admission. I come to this from a theoretical viewpoint… what you really need is to hear from someone who runs printing presses!
But I’ll toss in my opinion nevertheless.
If you want color results from offset printing that are predictable you must work with the printer. By which I mean “the print shop” / “the printing press operator” (not “the printer, that little box on your desk”).
Ideally, you would get a color profile from the printer that matches the press, paper and inks that are going to be used for the job, and use that profile as part of your process.
If you can’t work with the printer, probably the worst thing you can do is provide that unknown entity with a file that is in CMYK format. Why? Because implicit in the mixture of colors in the plates (layers) of a CMYK file are the characteristics of the ink, paper and press on which the job will be produce. If you don’t know those (and how could you? / why should you?), the results you get are going to be a roll of the dice.
In Photoshop, open the Color Settings dialog and click More Options. In the Working Spaces section, point over popup labeled CMYK and down at the bottom read the text in the Description section. Change the CMYK working space and now read different text in the Description section.
Notice up at the top again the setting for Gray and Spot “Dot Gain.” Do you know what those numbers should be for the press, paper and ink with which your job will be printed? Not likely. Let’s say you happily went along creating your Photoshop document while in CMYK color mode. It turns out that you were working on a poster that will be printed on a shiny surfaced heavy stock using a sheet-fed press. But the Working Space you were using happened to be “U.S. Web Uncoated v2.”
Well, unless the print shop manages to “fix” your file, what comes out of the press is going to be a long way from what you hoped would come out of it.
If you work in RGB, but convert to CMYK as the last step, you’ll encounter the exact same problem. With a flick of the wrist, you’ve changed the color mode, but done so using rules you are probably unaware of. And rules that almost certainly do not match those needed to “recreate” (as much as possible) the design you saw on your monitor.
For more “unpleasantness,” in an RGB file, choose “Convert to Profile…”. For Destination Space, change the popup to “Custom CMYK.” We see, at least, Dot Gain, GCR or UCR, Black Generation (for more fun, again choose “Custom…”), Black Ink Limit, Total Ink Limit, UCA Amount and a nice graph labeled “Gray Ramp.”
Now, depending on the ink(s), paper and individual press, any of those values or settings might change. Since it’s very unlikely that the person driving Photoshop will know these rather esoteric printing press details, I think that it’s just inviting disappointment to hand over a CMYK file to a client.
The phrase “out of gamut” was used in a previous post. That is certainly something to be aware of. But the implication is that the complete version would be “out of the CMYK gamut.” But there is no such thing as “the” CMYK gamut. Again, it depends on the ink(s) paper and characteristics of the individual press.
So, what to do?
Presuming that you use a high quality computer display, that is color calibrated (characterized)…then, if you don’t know where or how your job is going to be printed, probably the best thing to do is to deliver a file that’s in RGB format with an embedded color profile. Probably, a flattened file would be a good idea too (no additional channels or layers)… but you might want to leave text layers as text, in case the printer’s system can pull them out and rasterize them at very high resolution. Consult your printer.
When the print shop opens this file on their computer (which of course also has a high quality, color calibrated monitor), what they see on that monitor should very closely resemble what you saw on your monitor.
Now comes the magic. Since the print shop knows how they will be producing the job, they know how to properly convert your RGB file into a CMYK file that is tuned to their system. (And if they don’t know how to do that, you want to run out the door and find a different printer!!)
By the way, the reason I kept including “ink(s)” in my commentary above is because there is more than just a single standard formula for Cyan, Magenta or Yellow. Probably even black. Plus, there are some ink sets that, for example, specifically include a green (for the purpose of producing more vivid or more natural colors of foliage. So you may have happily converted your RGB file to CMYK, but on the printing press the color mix is CMYGK!
That’s it for me.