Supreme Commander

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

Background
For most of the Covenant's history, this rank was held solely by the Elite race, but was gradually mixed then replaced completely by Brutes after the Great Schism. They 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. Whether their status is lesser, equal to, or greater than that of an Imperial Admiral is up for debate, though the fact that Imperial Admirals command much larger numbers of fleets, while a Spreme Commander leads only one, indicates it is likely lower.

Physical Appearance
When commanding a fleet, Supreme Commanders wore violet armor with a cloak that wraps around and over the armor piece of their shoulders.

However, when the Arbiter was being tried for the destruction of Halo, he wore golden Zealot armor. Which could signify that Supreme Commander is one of several Zealot ranks, although this has yet to be confirmed.

The fact that the Arbiter wore the golden armor of a Zealot yet wore an unknown violet pigmented armor is quite odd. It could also prove that the creators/creator of that particular story part were exaggerating the Supreme Commander's appearance to allow the story to become a bit more interesting.

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 along with the fact that Plasma Rifles shoot green beams opposing to the fact that the original plasma rifles seen shoot cyan repeater-type bolts.

In the Halo Graphic Novel (main article)
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 usurped 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 wore Zealot armour when questioned by the Prophets, however, he could have been demote in that time.