Pantone to CMYK via ICC profile conversion in Illustrator

In the case of large format printers and the dedicated RIP applications that drive them, yes, it is better to leave Pantone spot colors applied to fills rather than attempt to convert them to a CMYK value.

The RIP will use its own look up tables to do the color conversion for the kind of printer it is controlling. For example, I can’t add manually light cyan and light magenta ink values to a CMYK mix to try to imitate a spot color like Pantone 485 C. I have to let the RIP do that. I also need to have a physical set Pantone spot color swatch books (coated and uncoated) to check the print output. It’s not enough to just look at spot colors on a computer screen.

Exceptions to this: items like gradient fills -don’t apply things like Pantone spot colors to a gradient, the printed results will be unpredictable. Use CMYK or RGB mixes for gradient fills. Don’t use spot color fills on anything with transparency fills/effects either.

Pantone Colours in the modern swatches are defined in LAB values, which are spectral readings of swatches of all the Pantone Inks they produce (Pantone themselves take these readings using spectrophotometers under very controlled circumstances and these are the values THEY dictate are the MATCH to their inks). LAB values are far beyond the colour space of any device we use and are “practically” the limit a human could actually see…  as such these LAB values are independent of any output device. Every device we use, whether it be a monitor or an offset press, inkjet printer, laserprinter, etc, has different colour responses. You have to think of Color Management as boxes of crayons. Some boxes of Crayola crayons have 128 crayons, some have 64 crayons, some have 24, some have less. Even if you have crayons from a different Brand that also have 128 crayons, their colours will be different than those in your box. If you choose a green crayon from your 128 box that your friend with just 24 crayons does not actually have, the best they could do is use the closeest green they have in their box.  ICC profiles define “how many crayons” a particular device has. All of these colours are read are stored as their respective LAB values.

So, in your situation. the ICC profile for the U.S. Web Coated (SWOP) v2 contains all the LAB values of all the possible colours a Web press printing in the same conditions CAN reproduce. A Colour management workflow will then compare the LAB value of your Pantone Colour, then find the closest match possible within the Web Coated ICC profile and use that as the CMYK values the press will use... to come as close to your Pantone Colour as physically. But every output device and paper combination is different, so this CMYK value you just determined will do something completely different on a different device, and it will. not. match.

Pantone’s Color Bridge System is Pantone’s attempt to give you consistent CMYK values as “standards” that can be specified, which they have tweaked to give as good a match with the simplest of CMYK values, but even these are based on a certain print situation and press calibration. They will work for “most” output in more modern print CMYK workflows, but will also suffer if output on devices outside that narrow gamut. Because there are so many different possibilities, the Bridge system has pretty much fallen out of favour. 

So, sure, you can go ahead and create swatches with CMYK values all you like, but don’t be surprised when they come up not matching.

You can check which profile is already there and if you want another one, without changing the current CMYK colors, you can assign another profile.

When selecting CMYK in the Swatches options, it will always use the document profile for the conversion.

Illustrator will use the profiles when converting spot to process. You want to have process as a result, right?

You can do it as Ton suggested, but I thought you want to have the fastest way.