Just look at the Channels Panel when you do the operations.
In CMYK equal amounts of Cyan, Magenta and Yellow do not appear neutral but brownish.
And the GCR (or UCR) settings of the CMYK Colour Space/ICC Profile determine how neutral RGB/Lab/… pixels will be translated to CMYK.
One of the reasons not to separate until the image is finished and do corrections in RGB (or Lab).
I try to desaturate an image
In CMYK Hue/Saturation with -100% Saturation will result in identical CMY-values and with »Colorize« checked CMY will become white and only K will have darker pixels so either option would cause problems.
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The idea is to do this in a “well behaved” colour mode, then convert to a high GCR CMYK profile and let colour management do the work of creating the neutral tones at the correct lightness value from light to dark.
Unless very specific CMYK colors have to be matched or if certain graphic elements are involved editing photographic images in Photoshop in CMYK is generally best avoided.
That is the “conventional wisdom”, which works for the majority, allowing the mavericks who disregard “what is believed to be known” to stand out from the masses!
Editing in CMYK is not that much of an issue for full colour/tonal range images if one has some knowledge/still/experience… However for four colour grey work it is probably taking things a bit too far (not impossible, just more work than the pay off is worth).
If the Mavericks can distinguish between what is known and what is believed to be known – good for them!
Thank for your answers.
In general, I do the first retouches in RGB. After that, I convert it to CMYK (fogra39) with the perceptual mode and refine the image and in the end, apply a especific ICC CMYK profile for a 8 colour plotter and correct minimun details (...I always use GCR... my substrate demands it). RGB to the final ICC gives lots of mistakes indepently of the conversion method... The plotter has a reduced gamut.
In the last profile, the difference between greys is terrible. In the image, you can see how the HighPass filter looks like.
I will try to create news ICC profiles (both RGB as CMYK) until I achieve it.
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Many operations in Photoshop only work correctly in RGB mode, they may not work in Lab or CMYK modes as they do in RGB mode.
Something does not sound right, however…
If I am understanding the situation correctly, you have issues with the “standard RGB ICC workflow” converting from a wider gamut source profile direct to the inkjet destination profile (CMYK profile that is wider gamut than standard press CMYK). You are using a perceptual transform from the wider gamut RGB to an intermediate press CMYK working space for final edits, before going to the inkjet device CMYK.
Why not just perform all edits in RGB, then convert to the F39 press CMYK profile, then to the final device profile if this is what works for you. It is not so much the edits in press CMYK space, but the conversion through press CMYK to the final profile that works for you.
As an alternative to using a standard offset press CMYK as an intermediate space, you could try using the following “PhotoGamut RGB” profile to convert from wide gamut RGB to this profile, then to your final device profile. This RGB profile has a wider gamut than standard offset press conditions, if also offers a perceptual transform from the source into PhotoGamut RGB and it is “well behaved” so R=G=B which will allow you to use high pass, desaturate etc.
Here is the PhotoGamut RGB colour space (ghosted) compared to Fogra39/ISO Coated v2 (colour):
It help me so much!