Found the solution on a different Adobe forum and it worked. Now, since 2012 when I started using Lightroom as well as Photoshop, my normal workflow has been to import images into Lightroom, do preliminary adjustments, then edit further in Photoshop. Then I print (or try to print) images through Photoshop. But with the cast problem still there, I decided this time to try printing from Lightroom. No difference. Then I looked more closely at the Lightroom printer settings. Good grief - they were set to allow the printer to manage colours. Because I usually print from Photoshop, I'd never looked at these settings before. So I corrected the printer settings in Lightroom and - OMG - perfect print!!!! I then printed from Photoshop, using a different photo but now with the Lightroom and Photoshop print settings set correctly. Again, a perfect print. So, I can only assume that Lightroom embeds print instructions into the images that are imported into it, regardless of whether the LR print facility is used, and that these embedded instructions are then not overwritten by Photoshop but sit alongside any Photoshop print choices that you make. And the result? A clash of colour management instructions and that nasty, nasty magenta cast. So, to conclude this long-winded story: if you use both LR and PS, make sure both have the correct printer settings, regardless of which program you print from.
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