Hi,
Firstly, when wide format guys ask you to submit files in CMYK - always ask them why. If they give you some convoluted answer about the printer being CMYK and colours always changing if the file is RGB or contains RGB elements be very wary indeed. The issue is that the final colour conversion the RIP software that runs the machines has to do is convert the colour into the output space of the individual device/media. If you're designing in a big CMYK space or as big RGB space makes no difference - if they can't convert from RGB to their output space accurately, they can't handle CMYK accurately either! What they're really saying is that the potential for you to specify colours they can't print is less if your artwork is in a CMYK space, which is normally true. But know what you're dealing with.
Suggesting Photoshop for a big banner is unusual. Not necessarily a bad idea depending on the size of your banner. If you create a file in Photoshop at 75-100 ppi at full size, that'll be sufficient. Provided you understand colour spaces and how to move colours between packages, supplying them a flat raster image with embedded ICC might well be your safest route. If this isn't something you do a lot and spending time coming up with a better vector workflow isn't worthwhile, then despite the huge file size, it's a quick solution.
Certainly if your vector image contains transparencies over Postscript Pantones your file would be flagged as potentially problematic because of the way RIP software handles those elements separately. Where Pantones aren't underneath transparency layers the best way to specify accurate Pantones is using Postscript. (see this link for detailed stuff about how these are handled in a print shop running Onyx -
Using Spot Colours for Great Pantone Matches | Hudson )
If you have a good vector file, Pantones specified as postscript spots, colours all ICC managed, then in my book that's what you send to the printer. You can't give a clearer instruction as to the requirement. Is it not the printer's job to determine how best to give those instructions to his machines so as to get an accurate result?
BUT (and it's a big one) if your file contains complicated elements and isn't saved so as to share the information accurately. eg. you save as a non colour managed PDF or an eps without postscript labelled colours then the printer has an uphill struggle.
As always in these situations it's about communication between customer and supplier to ensure instructions are accurate and expectations realistic. Then there's nothing to be nervous about.
Regards,
Craig
