Fatigues

Fatigues is a combat suit worn by military personnel (such as the UNSC Marines and Army) during none-combat duties. Though the role of fatigues might of changed through military time, seeing that the Marines during the Battle of Installation 04 worn them even in combat against the Covenant Troops. Though a likely reason for it to be worn is for its flexibility and light weightiness in combat, another reason is that when the Pillar of Autumn was attacked, Marine personnel probably had only minutes to gear up, so they most likely went on what thay had on there backs. Missions such as the one lead by Captain Keyes to the Weapons cache where the Marines of 2nd squad wore Fatigues in the swamp area, Though for this, they probably worn it for the permitting humidity swamps give out.

Effectiveness
Though worn under the main armor of the UNSC troops, without the extra protection, it can give more flexibility along with light weightiness (seeing that the armor is off). Though it comes with a disadvantage, it can prove fatal when shot from any weapon with any projectile, seeing its only made up of softer matter.

In-Game
Halo: Combat Evolved is the only game to feature UNSC Marines wearing Fatigues in Combat, all other games does not. Also, being that of a game, the Marines wearing this uniform can take the same amount of hits from any weapon, Covenant or Human wise.