I can add a few thoughts, as the ArchiCAD-to-Illustrator workflow is one I spent a long time perfecting both for on-screen [RGB] and print [CMYK] output, with a need to match specific paint chips, brick tones, and occasionally Pantone chip colours - note colour key at bottom right of image.
Some more of my ArchiCAD-Illustrator example images
Read point (1) through carefully please - it's critical to your question.
1) ArchiCAD (as Joojaa has correctly commented) has no intrinsic CMYK support, however you can define the specifics of a given pen colour in the pen tables to whatever you need, and there can be dozens of pensets in an ArchiCAD file, so you can specify one to a given RGB that you already know has a decent CMYK conversion, or at least one you've typified with your output device. This means you can have pensets in ArchiCAD which are effectively colour palettes aimed for use on a particular device, in the same way you output graphic design with a specific colour profile embedded for the target device it you're working at that level of control. Thus, even though ArchiCAD has no intrinsic CMYK support, it functionally doesn't matter if you take the time to typify your best conversions. There are pre-made GDL objects which output the entire penset as a colour grid on a test sheet, which will help you do your conversion testing.
Remember, originally, ArchiCAD pensets were matched to the specific pens of output plotters, back when large-format output was using literal pen-based plotters - you can use pensets as a major power-tool to help you manage print output differentiated between many different devices with care and time invested.
2) As Wolff pointed out so correctly in their comment, Illustrator's Select Same tool makes bulk colour conversions so simple and easy that you don't really need a lot more than that.
3) ArchiCAD DWGs will work fine in Illustrator, as will DXFs, but they will be just as segmented as the PDF linework, and cannot carry colour or greyscale properties well, and so cannot, for example, bring shadows in elevations, whereas the PDFs can.
4) In my personal ArchiCAD-to-Illustrator pipeline, I output from ArchiCAD via PDF, as I get best scale control that way, and can carry colour and greyscale info without trouble. I typically bring the PDF in, explode it, and work with the native elements inside Illustrator. I use the Select Same to choose elements with the same line colour, and move all linework to its own layer for ease of use, and bring that layer to the top of the draw order. I collect all shadows, put them on a layer, turn them all hard black, and then put a transparency on that whole layer of 33% and multiply, then put it second from the top in the draw order. All other elements, fill patterns (which have lower lineweights) colour fields, entourage, I draw or select and move to their own respective layers for ease of management - and name each layer clearly in plain English - not using CADD abbreviations. When next this Illustrator file is worked with, in weeks or months or years, it might be the intern using it, or the marketing director - or you, after so much time that your conventions have changed - so be clear!
You have a crucial early workflow choice - you can open the PDF in Illustrator and thus then edit native elements, or you can place and link it in; if you do link, you cannot, for example, change colour models of the underlying info, as you can't edit the linked info at all. One thing to bear in mind is that you can start linked, and later if more in-Illustrator control is needed, embed and explode.
When you do decide for a given project that you want to keep it linked to allow for design iterations, you draw your areas of colour and shade natively in illustrator, and you don't export those from ArchiCAD (Layers and Graphic Override settings) and you then place the linked PDF linework at the top of the draw order - it's great for iterating (as long as no-one changes the ArchiCAD sheet, or the view on the sheet at all - so have the BIM Manager lock those) as each time the design changes, the PDF is published as part of the milestone publishing process, you open in Illustrator and can instantly see what needs alterations, as now your colour blocks and shadows are misaligned with the linework. It's a great workflow for early design iteration, but can be limiting once you want to make specific graphic or æsthetic tweaks in the Illustrator file which you can't do in ArchiCAD - line texture / brushes, stroke weight dynamics - at that point if the design is not iterating as much or as quickly, it make be time to embed and explode the linework.
Hope this helps.
