GCR

I personally would not use or recommend Custom CMYK unless in a very specific situation. The Custom “legacy” CMYK engine is very powerful and flexible – however it is not based on any standard or modern condition which can lead to unexpected results unless the user is extremely knowledgeable.

You should get very good results with the following profile, even if the end conditions end up being GRACoL or SWOP rather than the intended Fogra39/ISO Coated v2 (yes, the GCR is that heavy for neutral tones!):

coated_FOGRA39_GCR_bas.icc

If the source RGB images are low key, then you should take care to create as much contrast as possible for “dark images” and err on the side of caution in making the darkest point that requires detail a little bit lighter just in case you wish to guard against the shadows plugging up too much. So make good use of tonal edits including curves and the shadow/highlight command and keep in mind that you can always have the RGB a little bit lighter, convert to CMYK and then add extra tweaks to the K channel (taking care not to push the total ink limit).

Good luck!

I take it that you want black & white images to print neutral, without any color cast?

In my experience, this is practically impossible with a standard cmyk profile, and you're on the right track with the GCR.

(D Fosse - are you saying that you're getting neutral results printing B&W images with a standard profile?)

You have to discuss this with your printer, but setting Black Generation to Heavy should make the job easier.

This will use more black ink, and less CMY, decreasing the likelihood for color casts. (see the difference between Medium and Heavy in the screenshots below)

If your printer does a lot of this kind of work, they might even have custom profile for this.

This kind of work requires close collaboration with the printer - do not use Custom CMYK without talking to the printer first.

If the printer can't help you, or doesn't know what you're talking about, find a different printer.

Also, do a test run first, and be present during the actual printing.

Although the print may look neutral when viewed with the 5000K light by the printing press (or daylight), it may look very different when viewed under any other artificial light - fluorescent or incandescent. (this is called metameric failure, and a magenta cast is very common)

So it's very useful to have a different artificial light source available close to the printing press.

https://forums.adobe.com/people/Humberto+Pinh%C3%A3o  wrote these images are constructed in cmyk

You may actually have a small problem already, because if you created this using a different CMYK profile than the one that will eventually be used, results may not be optimal. CMYK to CMYK conversions should always be avoided for a number of reasons.

It's always best to work in RGB and not convert until the end.

Even if you do have the right profile, you need to be careful to not exceed total ink limit. This is again built into CMYK profiles, so a conversion from RGB is always safe. But any subsequent editing may put you over unless you're careful. The result can be smearing and drying problems.

Gray component replacement is built into current standard CMYK profiles. Since GCR both saves ink and cleans up areas of high ink usage, I would think it's already dialed up as high as practically possible.

My advice is just to make sure you get the correct profile from the printer. Here in Europe most presses are calibrated to ISO standards, and a profile like ISO Coated (ECI) 300% is usually very accurate. I have never noticed any color balance problems - and I send a lot of work to offset print, most of it color critical.