About username colours
In trans/cross-wiki discussions, I would just consider it normal to have the username colours follow the same conventions as any other wiki links, i.e. distinguishing between local and remote wiki, knowing that these notions depend on the perspective since the same topic may be viewed as part of boards on multiple wikis. Honestly, I didn't even think of a risk to consider some users as "not part of the community"... An interesting point, indeed.
If that's the reason to make all usernames appear the same, then maybe the colour dedicated to usernames should be significantly more different from other wiki links. Not because it's useful to usernames themselves but rather to avoid breaking the convention established for other wiki links. But does it even make any sense to point to a certain user page? Which one to choose?
I think that Flow should be built for a target situation where each user would have just one global user page instead of one user page per wiki. Probably these would then be stored in a dedicated wiki. Keeping the established wikilink convention, the username colour would then be "remote blue" anywhere, except on user talk pages (stored on the dedicated wiki for user pages) where it would be "local blue" for all usernames. Alternatively, a dedicated username colour could still be picked as in "another approach" above. In both cases, the risk to view some users as "not part of the community", vanishes.