I can't see green as another service color on a starship as the captain wears it as his more casual and his more formal uniform color.
Personally, I prefer the Fletcher unis idea that Captains and higher wear white as their individual uniform particularly for formal wear (the tuxedo uniforms of the 2370s & 2380s also follow this trend). The 5-6 colour system used in TMP seems to be in improvement (Engineering & Maintenance, Security & Services, Bridge/Flight Operations, Scientific Research & Technical and Medical & Health Services)
I always associate "blank" grey as a cadet color, as we saw cadets on most series in it.
It's interesting the shades of colors they choose. For yellow they went with a mustard gold. For brown they went beige. I wonder if they needed a purple if they'd go indigo.
I agree on grey uniforms for cadets and trainees.
I think it depends on the shade of other uniforms: deep colored uniforms like most of post-2350 variants or the KT version of the TOS unis, I would say that indigo/violet/deep purple would probably work best. But paler/brighter uniforms like the
Cage/WNMHGB, Classic
TOS or the "Monster Maroons" would work better with a paler colour like lilac.
IMHO they should have Starfleet Marines handle all shipboard security.
I agree that security should be dedicated function, however I would split internal security/investigations (Starfleet Security, squad-level, primary type in canon), small unit direct action/base defense (Starfleet Special Operations, platoon or company level, see "The Seige of AR-558") recruited from
within Starfleet proper; and rapid deployment infantry/heavy weapons (Starfleet Marines or as I prefer Starfleet Ground Forces (TrekLit canon) & Starfleet Infantry Reserves) who mostly have a seperate chain-of-command and IMO are probably eld at strategic locations through out UFP space and aren't typically stationed on most starships (apart from the largest with sufficient warning). .
The US Navy aircraft carrier flight deck crews have numerous different colored uniforms for different operations; i.e. Purple is fuel, Red is ordnance, Yellow is plane director, etc. I wonder if Starfleet should have followed some of this tradition for their uniforms?
I'm not convinced that that would make sense.