Mass-effect – Where is all the Brute’s base armor

Like in Mass Effect 2, each individual weapon has different base damage values against Health (Red), Armor (Yellow), Shields (Blue), and Barriers (Purple).

The information linked to in the post is true for the general case - most Pistols deal the most damage to Armor. But you have variance between the individual weapons as well - the Arc Pistol is a pistol that specializes in dealing maximal damage to shields, not armor.

The exact benefit against any given defense depends on the individual weapon. (Melee and the various heavy melee abilities can be considered as weapons in this case)

That clarified, Armor, Shields and Barriers provide absolute protection until they fail (they are effectively a separate health bar), barring special abilities suck as the armor-piercing weapon mods (which allow some damage to 'penetrate' armor), and armor piercing ammo (same).

Any projectiles which benefit from the piercing (in this case, penetrate a .5 meter thick object) will deal reduced damage (60%). Projectiles which don't need to penetrate cover or an obstacle to hit their target do normal damage. This includes the visual armor that Husks / Cannibals can get, and the shields of Cerberus Guardians.

Additionally, depending on the type of enemy and the difficulty, some enemies have "Armor" (not to be confused with the yellow Armor bar some enemies have on top of / instead of health). This armor reduces damage by a flat amount, and it is this Armor that the piercing mod works against. So you do get some benefit when attacking all colors of health bar.