Hi alex.plt,
Thank you for contacting WebViewer forums,
I tested the same PDF provided on 8.3.3 and could not reproduce the color mismatching.
I also tested on 8.2.0 and there were no issues.
The colors are not related to an annotation as you can see that there is no annotation inside the PDF:
Perhaps you could try using color separation with guide here:
Here is a relevant forum post that might help with understanding the issue:
Best Regards,
Kevin Kim
Web Development Support Engineer
PDFTron Systems, Inc.