Starfleet’s gray uniforms, as depicted on the 2001 Sisko ornament, were introduced in Deep Space Nine’s “Rapture” (Season 5, Episode 10) on December 30, 1996.
This was the first episode of Deep Space Nine to feature the grey-on-black Starfleet uniform with the division color undershirts created for Star Trek: First Contact (November 26, 1996), which would be used for the remainder of the series. The uniforms had been held back in production so that they would not be seen until an episode that aired after the official release of the movie. Besides all the admirals’ (and Whatley’s aide’s) uniform which had not been changed yet, some officers of the lower ranking staff are still wearing the previous uniforms. Unlike the DS9 crew, the crew on Star Trek: Voyager continued to use the old DS9 Starfleet uniforms, due to being stranded in the Delta Quadrant. The Star Trek: Deep Space Nine Companion (p. 407) notes that all of Deep Space Nine‘s stock of the older uniforms were sent to Voyager to use.Memory Alpha
Our next clue is the publicity photo (above center) and Sisko with a bald head, goatee and wearing the older red-on-black uniform.
Prior to being cast as Ben Sisko in 1993, Avery Brooks played Hawk on Spenser: For Hire and its spinoff, A Man Called Hawk from 1985-1989. In both series, Brooks sported his preferred look: bald with a goatee. Paramount felt that fans would identify him as Hawk, so they had him grow his hair and shave his face for Trek (this was also done because they were worried about having two consecutively bald captains).
Fans, of course, still recognized Brooks as Hawk, because, wouldn’t you know, it was the same actor. Regardless, Avery Brooks wanted his look back and negotiated making changes over time, first growing the goatee back and then shaving his head. And—surprise, surprise—fans did not leave the series in droves.Screen Rant
So it appears that the ornament’s pose was inspired by a publicity photo for Season 4 and the uniform was changed to the gray-on-black design that viewers last saw Sisko wear when Deep Space Nine ended its run in 1999.