I've spent the last week trying to figure this out - and by all accounts should no longer be having this issue but here goes:

my office just migrated us over to 2018, and we decided to use it for a new project.

I have a pipe network for storm and sanitary in one drawing (networks.dwg), the storm network is on PR-Storm layer, the sanitary is on PR-U-Sanitary (in both the pipe style and the properties of the pipes). we then used data shortcuts to reference those into another drawing (base.dwg) - that drawing is then referenced into individual sheet drawings. We then use plot styles to control how things are printed. so in the grading drawing the storm should print black while the sanitary prints grey, while in the utility drawing the storm should be grey and the sanitary black.

Initially my issue was that the structures were printing in colour. A quick google search lead me to believe this was a known issue that had a hotfix issued for it (update 0.1). I contacted my IT department, she told me that I had update 2 installed and that it SHOULD cover all previous updates. However she also then found some articles saying that update 2 DIDNT include update 0.1. so she installed said update. My structures now print properly - but the pipes now print in colour. We've tried a fresh install with the updates installed in order (both on my machine and a completely new machine). I've tried changing the plot style to 'null' and the pipes still print in colour. I've opened a completely different drawing that doesnt use data shortcuts and have the same colour issue.

If I change the plot style of the layer that the Xref is on the pipes print by THAT plot style (which is great except that I need the two networks to print differently). they will not plot by the style set for the layer the pipes are on in the base file.

Any clues?