The only time we saw a person who worked for Starfleet Intelligence was the TNG episode "The Pegasus", where Admiral Blackwell said that Admiral Pressman would be coming aboard on behalf of SFI. We then saw Pressman, wearing red, with two pips on his collar.
White collars only existed during the TOS movies (and no doubt for an unspecified and unseen period of time after that, although by the 2340s all that was left were white shoulder straps, in TNG "Yesterday's Enterprise). There, they were worn by high-ranking people, including starship COs and admirals and eventually also Scotty (after he gained the rank of Captain). None of these characters was explicitly said to work for SFI.
On the subject of JAG, or the lawyers of Starfleet, it's true that a JAG officer in TNG (Philippa Louvois) wore red. However, a JAG officer in TOS (Areel Shaw) also wore red, the color that in TNG era would become yellow. Thus, either there's no JAG color (just like there doesn't seem to be any SFI color), or then the color changed between TOS and TNG.
As for Marines, well, ship's security in the TOS movies had its own color, dark green. But in ST5:TFF, a hostage liberation force was seen wearing dark blue instead, so we could argue these were not Security but indeed some specialist force - Marines, SEALs, Army, Police, SWAT, whatever. That's just TOS movies, though. In TOS, redshirts handled preplanned ground combat almost invariably, and weren't treated as separate from Ship's Security. In the TNG era, the same applied to yellowshirts. Except when we saw small snippets of ground warfare, in which several ground fighters were seen wearing red and some were seen wearing blue, despite supposedly representing a preplanned, dedicated ground fighting force ("Nor a Battle to the Strong", "The Siege of AR-558"). So we could argue that there's no color for Marines or other ground combat specialists as such, outside the TOS movies - but that doesn't mean that Marines or the like couldn't exist. They'd just wear the usual assortment of colors; a Marine commanding officer would wear red, a Marine technician would wear yellow, a Marine medic or sensor specialist would wear blue.
One interesting question is, if O'Brien was an infantryman of some sort back in the 2340s, under the command of Maxwell, why did he wear a non-yellow shirt? He says in "Paradise" that he only started to wear yellow after saving his fellow troopers through repairing a transporter on Setlik III, and earning the position of Tactical Officer on the Rutledge. What did he wear until then?
Timo Saloniemi