A grand admiral's uniform were worn by the Grand Admirals of the Galactic Empire and were a variant of the Imperial military uniforms, including by Thrawn and Rae Sloane.
Around two years before the Battle of Yavin, in the year 2 BBY, Grand Admiral Thrawn of the Seventh Fleet wore the uniform when he was tasked with defeating the early rebellion against the Galactic Empire, along with a black holster to carry his blaster pistol. The uniform was still in use in the year 5 ABY, as it was worn by Grand Admiral Rae Sloane, the public figurehead of the shattered Empire at the time.
Counselor to the Empire Gallius Rax, who acted as the de facto leader of the Galactic Empire at large after its fragmentation following the Battle of Endor, wore a white naval uniform—similar to the uniform worn by Imperial Grand Admirals—along with a red cape. Rax met his end during the Battle of Jakku, but he tasked Sloane with completing the Imperial Contingency, kicking off a path of events that eventually led to the formation of the First Order. Meanwhile, Thrawn continued to wear his white Grand Admiral uniform in his exile on the extragalactic planet Peridea, although it grew slightly damaged over the years.
Grand Admiral Rae Sloane of the Imperial Navy.
Similar in cut to the standard Imperial uniform, the Grand Admiral uniform consisted double-breasted white tunic, made of gaberwool, with a round collar and matching trousers, and durasteel-toed knee boots with turndown topside cuffs.
The uniform also featured several code cylinders, a black belt and an identity disk placed on the belt buckle; and a rank insignia plaque with twelve squares, three blue and three gold over trios of blue and red, which was also used by the Grand Generals of the Imperial Army. While Tarkin and many military leaders believed that the use of medals and battle ribbons put more emphasis on status as opposed to duty, the Grand Admiral uniform had gold epaulets.
In potentially hostile situations, Grand Admirals could wear battle armor over their tunic, as well as a protective helmet. The gear was also white-colored, with golden shoulder boards similar to the ones on their regular tunic.
Grand Admirals Thrawn and Miltin Takel as depicted in the Legends continuity.
The rank of Grand Admiral, and the uniform, first appeared in the 1991 Star Wars Legends novel Heir to the Empire, written by Timothy Zahn. The uniform was first pictured in the novel's cover, created by Tom Jung. At Star Wars Celebration London on July 16, 2016, it was revealed that both the title and the uniform would be introduced into canon with the respective releases of the third season of the animated series Star Wars Rebels and the 2017 novel Thrawn, also written by Timothy Zahn.
Although the character of Grand Admiral Rae Sloane was depicted wearing the traditional Grand Admiral uniform, the 2016 canon novel Aftermath: Life Debt suggested that she continued to wear an Imperial gray jacket after she had attained the rank of Grand Admiral.