From Halopedia, the Halo wiki
Captains are one of the highest castes of the Yanme'e, and rarely encountered on the battlefield. They can be denoted by their red-coloured carapace and - unusual for Yanme'e - energy shielding. They are also often equipped with Okarda'phaa-pattern plasma rifles (or for those individuals serving under the Jiralhanae, Brute plasma rifles), rather than the plasma pistols and needlers afforded to lower-ranked individuals.
Halo 3: ODST
Yanme'e Captains are rarely encountered in Halo 3: ODST. In Firefight, Captains can spawn alongside Yanme'e infantry (consisting of Minors, Majors and Ultras) but do not replace infantry - instead counting as "Yanme'e Leaders" alongside the gold-clad Captain Majors. Captains and Captain Majors spawn in a ratio of 3:1.
List of appearances