[macdawson]

Capt. Yamanami's colar was green, though his coat was the same as Okita and Hijikata (whose collar is orange, not red), only slightly shorter. Another difference is Yamanami, Okita and Hijikata's insignias and uniform details (cap shield, shoulder stripes, trim, etc.) are golden over black, Mamoru's are silver over dark grey. So maybe that's a captain thing. We never saw any other captains of a Yukikaze class destroyer or around the same rank level of Mamoru so this is a mere guess. But Okita's son has the same jacket (grey with silver details) as Mamoru, unfortunately there's no piece of his uniform showing so we can't say for certain wether or not it is orange also.

Oh, and just to add to the confusion, the guy that seems to be the captain of the battleship Abakuma (the one that gets hit by the destroyer Shimakaze), seems to be wearing a standard crew uniform but with a captain's hat instead of a blue cap.