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.