Converting RGB to CMYK canon Rules, and possible script

Hi @Fred.L , Also, the source and destination for an RGB to CMYK Color Picker conversion would depend on whether there are documents open when you pick the color. As Peter suggests the RGB and CMYK Color Settings Working Spaces would be used for a Color Picker RGB to CMYK conversion, but only if there are no documents open.

Here I have AdobeRGB set as my RGB Working Space, Coated GRACoL 2006 as my CMYK Working Space, and matching Conversion Options (which also affect a conversion), Relative Colorimetric with Black Point Compensation turned on. Note that Illustrator requires an open document to get at its Color Picker, so I have a CMYK Illustrator doc open with the Coated GRACoL 2006 CMYK destination assigned. The Color Picker conversions in the 3 apps match:

If there is a document open, the document’s assigned profile(s) are used—the document assignment could be different than the Color Settings Working Space, so you have to check Edit>Assign profiles....

InDesign allows you to mix RGB and CMYK colors in the same document, so an ID doc has both an RGB and CMYK profile assignment. Here I’ve changed the RGB and CMYK document assignments to sRGB and US Sheetfed Coated respectively, and you can see the color managed conversion of 100|100|150 sRGB to US Sheetfed Coated CMYK has changed from 70|63|13|1 to  66|56|9|0:

A CMYK profile handles the GCR and black generation on a conversion to CMYK, so you can reduce the amount of black by creating a profile for the press with lighter black generation. I don’t think there would be a way to script GCR for an existing CMYK color and keep its color’s appearance for the destination press.

As to why your conversions might be different among Adobe apps, you must be sure that both the RGB working sapce and the CMYK conversion space are the same across all the apps.

So, why isn't there an algorithm? It would certainly be convenient!

The reason is that CMYK isn't a colour, it's a recipe. Many designers are not trained in this. They are told to convert to CMYK because that's what prints. But here's the problem: different brands of CMYK inks are actually different. Worse, the same inks look utterly different on shiny magazine stock versus cheap grey newsprint. So the conversion to CMYK has to allow for those differences to make an appearance similar to each other and the original. That's colour management. That's why we have to set CMYK (and RGB) profiles. There is no case of "default" or "absent" profile.

Let's take 3 first. These results are junk. What, all the different web sites? Yes, junk. They are based on the myth, the lie, that there is an algorithm, a single correct conversion.  Made by people who understand web sites but don't understand Color management.