I just re-read the Cole Protocol recently, and I'm unsure whether the purple cloak was either custom clothing or the uniform for the so-called Supreme Commander.
As most of you who have read the book know, 'Vadamee wore a purple Ship Master's cloak. Doubtless, as his ambition was to become a Fleet Master, and that he was drafted into the Fleet of Particular Justice at the end of the novel, and that he became the Supreme Commander, and later, the Arbiter, I'm wondering whether Supreme Commander is simply a over-glorified term or honorific for Fleet Master.
In Halo 2, which is considered higher canon than the Halo Graphic novel (the artist of that particular story did take many liberties with canonical art, such as the appearance of the Supreme Commander), 'Vadamee had golden Zealot armor.
I think that Supreme Commander is simply another name and/or honorific for Fleet Master, and that some reckoning must be taken due to these new updates. The term Supreme Commander is extremely out of date. The author (and) artist also took liberties on names, such as Kusovai. Perhaps the purple armor and cloak were for ceremonial purposes, as the Fleet of Particular Justice was, indeed, a very special fleet.
I must also stress the importance of the Arbiter's name, and once and for all have solid proof that the Arbiter is in fact, Thel 'Vadamee.