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First i asked that question that question on facebook and it failed. Many didn´t understand that question correctly and told me about the basics about color-management and how that is a bad idea,......
that´s not the question.

If i let´s say have a AdobeRGB Document and choose a color with the cmyk-sliders. They are clearly limited and don´t cover the whole AdobeRGB colorspace. The question is to what colorspace are they limited?
I changed the default cmyk-colorprofile in the preferences but that didn´t affect the cmyk-sliders in rgb documents in any way, so it doesn´t seem to be based on that.
So what are the cmyk-sliders based on?

 

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I am not sure if I understood your question. Wouldn't you use the RGB siliders to specify colors in an RGB document, as the CMYK definitions cannot possibly be used to specify colors that are off the color gamut of the CMYK color space?

This is how the Color panel looks in an RGB document with Adobe RGB color profile when spefifying full 255 red fill for an object (but on the forum the full color gamut cannot be displayed):

adobergb_rgb.jpg.85f568699799904cdfb88285ec9e7294.jpg 

If you select CMYK from the dropdown list of the Color panel, you can see CMYK value conversion for RGB 255, 0, 0 calculated based on the active document color profile (AdobeRGB in this case), and last used CMYK color profile, which in this case was ISO Coated v2. If there is no last CMYK profile available, the app seems to use the default CMYK color profile defined in the Preferences > Color:

adobergb_cmyk.jpg.17e2e1085f0203775bee2a79008e5c4d.jpg

 

As the lock is on in the Color panel, the appearance of the color is also retained. If not, the object would be shown as if it were printed in CMYK color mode uisng the displayed CMYK values.

There is no CMYK equivalent color definition for an AdobeRGB R255, G0, B0 but what is shown is how RGB 255, 0, 0 converts (from larger to narrower color gamut) when the destination CMYK profile is ISO Coated v2. There is no going back so if you now use the CMYK sliders or boxes and even type the exact same color value to any of the boxes, an RGB conversion is done based on AdobeRGB profile for the displayed CMYK color definition (from a narrower to a larger color gamut) and you would not get anything even close to R255, G0, B0 values but completely different RGB values. The appearance of the color would change, as well, and there is no way you could specify a CMYK based color value that would convert to RGB 255, 0, 0.

adobergb_cmyk_redefined.jpg.f96cd4861f3d94c0cb1d9bf5ea95f25f.jpg

Perhaps you were referring to this with your question?

The point is, if you want to specify a color for an RGB output, use the RGB sliders (in either document color mode), and this color will be used in RGB exports (if last used RGB profile is not changed at the time of export) while CMYK conversions for CMYK exports are determined by the CMYK color profile at export time. And vice versa. It is recommended however that if you are going to print your document using commercial press, you have your document in CMYK color mode. This way e.g. the PDF/X profiles behave as expected. This way you can also guarantee that your black text (K100) will not convert to four-color (rich) black. Note though that when the document is in CMYK color mode the RGB definitions will also be shown in CMYK mode (as if in proof mode for printing), even if you still can produce full RGB gamut exports from the document (e.g. for photos, which you would typically have imported in RGB mode, and for those objects which have been defined with RGB values while keeping the Color panel lock on).

I am not sure about all implications of the lock in the Color panel but I always keep it in the locked position; at least it keeps the color values unchanged when you switch between the CMYK and RGB sliders -- until you make a new color definition; if the sliders are unlocked simply switching between the slider modes will recalculate the color values; if you keep the lock on, the app remembers the last used color definition and mode for an object and uses this as the basis for output in both color modes until the color is re-defined (and the conversion is recalculated), or until target profiles are changed.

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yeah it´s not the question "should i work this way?" No, obviously not^^
I asked out of curiosity "what is happening under the hood?".

Yeah the "lock colorspace" also wasn´t clear to me, for a longer time.

At least on my test it didn´t take into account the last cmyk colorprofile i used or the default cmyk in the preferences.
So let´s say i am confused.^^
I changed my default cmyk profile to "Japan Color 2002 Newspaper" which is very small and the K100 also ends up in this brown-dark grey.
And did create some documents in that colorspace and saved them and stuff. Still when i created a new rgb document and created a K100 Color which was way darker and the brown-tint is gone.
I created a ROMM RGB Document and put a bitmap with the newspaper profile in it. That colorspace is way smaller than i could create with the cmyk-sliders.

At first i thought they simply convert the values like these online color-converter: https://www.rapidtables.com/convert/color/cmyk-to-rgb.html
(which yeah are quite funny, but are waaay too simplified) But a simple tested proofed me wrong.

So i am at a loss. Krita for example asks for an colorprofile when changing the colortool to a different colormodel than the document colormodel.
That´s what i get, but i don´t get what is automagically happening here.

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10 hours ago, Gmit said:

I changed my default cmyk profile to "Japan Color 2002 Newspaper" which is very small and the K100 also ends up in this brown-dark grey.
And did create some documents in that colorspace and saved them and stuff. Still when i created a new rgb document and created a K100 Color which was way darker and the brown-tint is gone.

I could not reproduce this:

rgb_cmyk_japanese.jpg.1db3c7a00b4b3e1db09595fb4c1b47a5.jpg

The top layer is an RGB/8 document that is created while having the Japan Color 2000 Newspaper specified as the defaut CMYK profile (I restarted Publisher to make sure it takes).

The bottom layer is a CMYK/8 document with the Japan Color 2000 Newspaper profile (having sRGB and ISO Coated v2 as the default CMYK profile). 

Both show equally brownish appearance with K100 color definition. Many profiles for uncoated papers show this behavior and it is "by design" even if often a bit overdone.

10 hours ago, Gmit said:

That´s what i get, but i don´t get what is automagically happening here.

Color management is poorly documented in Affinity apps, and you do not get any feedback in the UI. But it works pretty much the way color management works in other software that can do both RGB and print. There needs to be a way to handle dual color modes and deal with multiple color profiles, with the support of both assigning color profiles, and converting from one to another. In addition to that, there needs to be support for export time color conversions. It can get complex and without documentation there is much confusion, guessing and need for running tests...  

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Maybe my computer needed an additional restart or so, to take on the new cmyk-profile. I did a quick retest and now it behaved exactly as you just described.

So, i would leave it at that, i already put so much time in that question and asking different people, checking what different apps are behaving,.... and all of that just out of curiosity.
Your explanation makes sense, and from the quick test it looks like that´s how it should behave. Unless someone proofs a different behavior.
I can´t say that this knowledge helped me work better, i don´t put cmyk-values in my rgb-document^^ But it made me more confident in my knowledge of the Affinity Apps, i know more what is happening under the hood.

And i choose that Newspaper profile to have a easy time to tell the different cmyk-profiles apart. Interesting you say that "if often a bit overdone" for that brownish tint. But i also to 99,99% of the time don´t work with newspaper-paper or similar, but good to know :D

Thank your for your time and input, most likely the best source of information in researching that question.


 

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2 hours ago, Gmit said:

And i choose that Newspaper profile to have a easy time to tell the different cmyk-profiles apart. Interesting you say that "if often a bit overdone" for that brownish tint. But i also to 99,99% of the time don´t work with newspaper-paper or similar, but good to know :D

I suppose it has something to do with trying to simulate printout on off-white wood pulp papers. It is sometimes meaningful also to make clear visual distintion between K100 and rich-black texts even if the difference were not that clear on paper (the richness of black is always relative and also dependent on inks and the media). E.g. in InDesign there is a setting that allows making this more accentuated on screen so that rich blacks can immediately be recognized. I think that brownish blacks that are present in profiles for uncoated stock are related to this.

Colors are complex, and intuitive, so confusion is built-in.

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