I've just noticed that any files or folders viewed from within a library virtual folder don't have any colour or doctype displays applied.
The same folders viewed outside the library root folder looks normal.
All the files are there, just no UI "enhancements" are visible.
I'm still figuring out how (or if!) libraries will work, given the fairly severe constraints imposed (in RC build 7100, only a maximum of 50 "root" folders can be added, but there's no limitation on how many subfolders are included with each "root" folder).
I think maybe I'll learn some more about collections, they seem to work a bit more consistently within Opus. I just haven't sat down with the help and read anything...
Meanwhile, this is just FYI, and I apologise if I'm misunderstanding something fundamental about DO.