What Starfleet uniform should Seven of Nine wear?

[Gaith]

most of [the Maquis] were former Starfleet, iirc.

Perhaps, but not Lon Suder.

In the episode Human Error when Seven created a holodeck simulation of her living a regular life, she had herself wear a blue Starfleet uniform in the simulation, so therefore if she had worn a uniform while serving on Voyager, it would have been blue.

Indeed.

For regular Voyager I would say normal science outfit. Otherwise, I'd have her wear the Borg outfit for about half a season and then have her wear something original and more normal.

Also this. Neelix and Kes didn't wear uniforms because, though they were crew members with jobs, they were outside the standard Starfleet command and rank structure, more akin to independent contractors. In not wearing a uniform, Seven was theoretically closer to them than former Maquis like even Lon Suder, even though her independent contractor job in Astrometrics was of much more tactical and strategic consequence to the mission. Ideally, however, she would have worn more of a utilitarian jumpsuit, more like the NX uniforms with pockets and such, than the form-fitting catsuit.

 

[SpockOfNine]

She would wear teal/blue for Science. Neelix would be in yellow/gold.