From Halopedia, the Halo wiki
Fatigues are part of the battle dress uniform worn by military personnel of the UNSC Armed Forces during combat duties. Rather than full-body combat armor, like those worn by the ODSTs, regular fatigues are relatively simple cloth garments, with the benefit of flexibility and light weight, though at the cost of less protection from enemy fire.
Though fatigues provide very little actual protection, they can be covered with ballistic armor plating, such as the M52B body armor, often complemented by various armor sections in the legs, shoulders and forearms. UNSC fatigues typically include camouflage to allow the wearer to blend into their environments. Camouflage patterns utilized by the UNSC include a gray "urban" color, a mottled green "jungle/forest" camouflage, a light grey/white "snow" camouflage, and a desert pattern. UNSC Air Force pilots wear fatigues with a maritime blue camouflage pattern. While not nearly as effective as Covenant active camouflage, or the photoreactive SPI suits used by the SPARTAN-IIIs, it provides users with a basic level of concealment from enemy forces.
Fatigues are worn on missions where the added protection of heavier armor is a disadvantage, such as difficult terrain or for stealth missions. The clatter of armor may alert previously oblivious enemies to their approach, and the weight and bulkiness of the armor may inhibit the wearer's combat performance.
Likely to consist of normal cloth, rather than heavy armor plating, fatigues provide little protection, especially from Covenant directed energy weapons, which can melt and burn through most materials easily anyway. Advances in fabric materials and manufacture may allow the cloth a Kevlar-like resistance to projectile weapons, or high durability, but its chief advantage lies in its light weight and easy of movement.
Halo: Combat Evolved is the only known game to feature UNSC Marines wearing only regular fatigues, without the overlaid armor plating, especially in the mission 343 Guilty Spark. This does not alter their abilities, but non-armored Marines can take only half as much damage as armored Marines.
Halo Reach features various camouflage schemes on fatigues worn by UNSC Army troopers. The updated Marine BDU in Reach features an urban pattern as opposed to the solid grey/blue from Halo: Combat Evolved.