According to the EU novel Star Trek: Enterprise - The Good That Men Do, after she received her commission from Starfleet T'Pol continued to wear a modified Vulcan uniform because she wanted to (it representing a link to her former life as an officer in the Vulcan forces). In turn, Archer was happy to tolerate this because of his respect for her on a personal level and presumably seeing no reason to push the issue.
T’Pol turned and picked up one of Trip’s royal blue uniform jumpsuits.
After the Vulcan High Command had cashiered her, Starfleet had granted
her a commission. Yet she had never donned their uniform. Perhaps the
Vulcan uniform she still wore—a garment that now bore Starfleet
commander’s pips—represented an illogical attachment to the past.
Trip's death she evidently reconsidered her position.
"And perhaps Trip’s death signified that the time had finally arrived to move past such impulses."
T'pol chose to switch to wearing a Starfleet uniform in his honour, his death representing the point at which she began to fully accept that she was actually in Starfleet and that it was time to stop thinking of herself as an outsider. Note that Archer is vaguely bemused by this decision rather than anything else.
The captain recalled how he’d felt four years ago, that he didn’t want
a Vulcan on his ship. Now, he couldn’t imagine Enterprise without
T’Pol. His science officer, quiet, competent, and still able to
surprise her captain. This morning he stepped out of his ready room
and immediately noticed that something was off. Looking towards the
science station, Archer saw T’Pol in a Starfleet uniform. Even now he
had to suppress a smile. Turning toward the science station, Archer
asked, “T’Pol, have you found any ships in the vicinity that might
correspond to the warp trail we followed here?”