The way I've been interpreting things:
"Simply" ignore whichever aspect of the target that is affected by the spell. Either totally or in proportion of the importance of the effect.
Sword with pink dot: treat as if the sword didn't have a pink dot. You are hit by a sword.
Magically sharp sword: treat as if the sword was not sharp at all. You are hit by a blunt object.
Magically dulled sword: also treat as if the sword was not sharp at all. You are hit by a blunt object.
Magically created sword: ignore the sword completely.
Invested device: The Verditius Magic indicate that a device must be able to mundanely hold its weight, from which I infer that it is "structurally" mundane, and thus you can still be hit by them. Active invested effects are considered individually for effect and penetration.
Natural weapons of magical creatures: Hit normally to the extent that they aren't magically enhanced (i.e. they aren't listed as Powers).
Poison turned into milk: non-poisonous, but neither nutritious nor even tasting like milk.
Shrunk boulder: ignore anything that's related to size and weight.
Rego-ed projectile: ignore the rego effect (i.e. 'bounces off').
Chamber of Spring Breezes: ignore the wind effect.
Poison gas: ignore the poison. I don't know what the medieval paradigm is on gases. I assume there is no notion of "displacing" breathable air, so the
Parma would only act to neutralize the harmful vapors.
Falling on a Creo-ed stone bridge: you take damage from your fall being stopped (as was mentioned in another thread), but you don't take damage from scraping against the rock.
Think of it as a Perdo effect that dulls the qualities that have been magically affected to the point that their effect on the magus are negligible. I think it also works well with that blurb I remember reading (4th ed?) about how some magi didn't like keeping a Parma up at all time because it stopped them from experiencing the world in its fullness. If you follow the interpretation that the Magic realm is the origin of the platonic forms, the more perfect something is, the closer to the Magic realm it is and thus the more it would be "dulled" by Parma.
Note that this only changes how the spell's target affects you, not how you affect the spell's target: Parma doesn't enable you to do something you couldn't do otherwise (note that I don't say "couldn't do safely"). You can't pick up a pebble that has been turned into a huge boulder, you can't walk through a Creo-ed wall, etc.