Does your RIP have Adobe PDF Print Engine (APPE) capabilities?
If so, I'd still try to get across to clients that they shouldn't send spot colours (I'd guess you're already doing this and appreciate that you'll never get through to all of them!), but start recommending PDX/X-4, switch APPE on in the RIP, and let it do the conversions. APPE should render spot colour transparency correctly.
Hi Danny, thanks for the reply.
I'm not sure about the RIP, will look into that. The spot colours are not such a problem though, as we can identify these - the issue is when it happens with objects not containing spot colours, as we're unable to spot these in advance!
Using archaic standards like X/1-a is one very very big reason. I won’t pretend to be a color management expert but flattening transparency in this way is a horrible recommendation.
You should be recommending and accepting PDF X/4 which is not device dependent and won’t flatten transparency.
YDB is caused by RIPs that handle continuous tone rasters independently from vector art (and I think that was explained in the article about YDB on InDesign Secrets). You typically would see it when you have some sort of transparency on top of a native vector object, like a color-filled frame drawn in ID.
Frankly I think you are shooting yourselves in the foot by requiring pre-flattened files since you cannot correct YDB once the file is flat.
So, my query is this: Does anyone know what exactly causes the discoloured boxes,
Your requirement of PDF/X-1a forces transparency to be flattened on export and the symptoms you are describing are likely caused by the client's chosen flattener preset and/or transparency blend space. PDF/X-1a also forces all color except for spot color to CMYK, so your requirement forces your client to make CMYK conversions (do they know what profile to use), and flatten transparency (what flattener preset and blend space should be used); and at the same time allows spot colors which shouldn't be used for a process job.
What's the rational for requesting X-1a? If you are trying to avoid making color conversions on your end you still have to convert spot colors, which can be very difficult when there are tints or transparency involved. You could recommend Pantone Bridge to clients who want to use the Pantone system for process jobs—those colors are CMYK process, not spot.
Thanks for your help everyone, this is really useful - I think we're going to go away and have a good look at the way we're currently processing our files.