UNSC Navy working uniform
From Halopedia, the Halo wiki
Working Uniforms are part of a uniform (much like fatigues) worn by military personnel in Non-Combat situations, such as the crewmembers aboard the Pillar of Autumn[1] and UNSC In Amber Clad. They provide little to no protection, only offering a covering specifically for their jobs and tasks at hand, such as Engineering and Tech support on ships or groundside.
Purpose
These uniforms offer only little protection and are not meant for combat personnel. Though they do come in a variety of colors such as blue, red, orange, purple, and yellow, as seen being worn by the crew in the "PoA". Such colors differs for what job they do, though they are not known for sure, personnel wearing orange, blue, and red can be seen operating computers on the bridge, cryo bays, and anywhere else. In Halo 2, they were rarely seen, only the crew of Miranda Keyes' had them, though, the color gray was introduced and was worn by the ships pilots, but no one else[2]. In Halo 3, they were seen yet again in the level Crows Nest near the beginning, operating portable computers, security cameras and other tech[3].
In-game
As seen in the level Pillar of Autumn, crewmembers had the ability to arm themselves, possibly being the ships "Security Force" when the Combat Personnel aren't present. Though, for those who fought, a few shots from a plasma bolt can bring down a crewman, seeing as the Uniform they wore weren't effective against the Covenant.
