Damage types

The color of a projectile or attack is a reliable indicator of what things it can damage.

Orange (or Yellow) damage applies to everything fired by the player. It only damages Enemies and not other players.

Blue damage applies to most enemy attacks (eg. most bullets, Maser Turrets, Banshee Spectres). It only damages players.

Red damage applies to some enemy attacks (eg. Blaster Titans, Mines, Laser Turrets, Reaper Spectres), and some environmental hazards (eg. Heat, Convection). It damages both players and enemies alike.

Miscellaneous
Purple bullets (ie. Venom Phase 6, Medulla Phase 5) deal 6 damage rather than the usual 1. This makes them an instant knockdown, unless you have Accelerate, in which case they deal 4. This means you can survive one hit from full health.

Dark red bullets are fired by Liquorice in Collider. They deal 4 points of red-type damage to players and 10 to enemies, and are more (but not completely) resistant to Plasma Shield.