Supreme Commander

Supreme Commander is a very high Covenant Elite (Sangheili) and Brute (Jiralhanae) rank.

Background
For most of the Covenant's history, this rank was held by the Elite race, but was gradually mixed then replaced completely by Brutes after the Great Schism. They now command Fleets and Armadas.

Rank
Supreme Commander is the rank which commands a Covenant Fleet, such as the Fleet of Particular Justice formerly led by the Arbiter. Their armor color is usually purple.

Physical Appearance
When commanding a Fleet, Supreme Commanders wore violet armor with a cloak that wraps around and over the armour piece of their shoulders. In fact, the Supreme Commander's unusual helmet looks very similar to the Halo 3 Elite combat helmet when compared closely.

However, when the Arbiter was being trailed for the destruction of Halo, he wore golden Zealot armour, which means that the rank of Supreme Commander is a rank that can be obtained by a Zealot.

The fact that the Arbiter wore the golden armour of a Zealot yet wore an unknown violet pigmented armour is quite odd. It could also prove that the creators/creator of that particular story part was exaggerating the Supreme Commander's appearance to allow the story to become a bit more interesting. Alternatively, it might be used for separate occasions, such as his hearing or a military objective. To prove this theory generally in Second Sunrise Over New Mombasa, the Elites (Sangheili) and their weapons appear to be exaggerated also. The Sangheili Energy Swords are pink and Plasma Rifles shoot green beams opposing to the fact that the original Plasma Rifles seen shoot cyan/blue repeater-type bolts.

Appearances
The particular Supreme Commander in the aforementioned text was a friend of, and yet superior to, Rtas 'Vadumee and would later become the Arbiter. During the Civil War of the Covenant, it is possible the Brutes took these positions from the Elites. But even so, some Supreme Commanders escaped to fight the Brutes.

Trivia

 * The future Arbiter was a Supreme Commander, yet he was seen clad in Zealot armour when questioned by the Prophets, however, he could have been demoted in rank by that time. Another theory is that he wore this armour to show the Prophets they still had superiority and control over him or it might have been due to the ceremonious atmosphere.