From what I understand, the red uniform covers all personnel who control ship functions or command. Tom Paris in Voyager had no subordinate staff, but as the helmsman wore a red uniform. Similar is the case of Worf in TNG season 1 (I guess as Tactical Officer his role was a control one).
I think also as in the real life armed forces, those officers who stand out become heads of departments, XOs and COs.
I'd agree that Red tends to lend itself to Command, but more importantly ship functionality.
That being said I should point out that Tom Paris was the senior Conn officer and as such had "staff" in so far that he was directly in charge of implementing ship course data and maintaining the logs and uniformity of all junior conn officers. We've also seen that he coordinated with Astrometrics and Engineering regularly where propulsion and navigation were concerned.
They seemed to show Paris as what one might expect as the lead / chief Conn officer to be. One then does begin to question why we saw Wesley Crusher at the Conn so often. Even a field promotion to that position very likely wouldn't make him a the lead / chief Conn officer. The only in universe explanation I can think of is its possible that a galaxy class ship has so much support staff for that position that a lead isn't a required position on that class of starship. (Not sure I buy that one!)
Riker was XO on three ships before taking his own command, so Sisko's service on two ships as XO makes sense. As for the Saratoga being a step "down" from his previous service on the Okinawa, well, we don't know what class the Okinawa was, and there's no reason to think that a Miranda-class vessel is less prestigious than an Excelsior-class.
According to Memory beta, Riker was only the XO on one ship before the Enterprise, but I DO very distinctly remember one other being mentioned in series.
Either way, we do know Riker, like Sisko, was the Chief Engineer of the Hood (at least that was very heavily implied) before he moved onto XO.