Just an idea that would really simplify Parma
Creo IS resisted, so magically created things like Ball of Abyssmal Flame would have to penetrate.
Intellego IS resisted, making it hard to gain information about a magus.
Muto IS resisted, as are muto effects that would enhance an attack aimed at the magus, such as Razor's Edge. The sword it was cast on would still harm the magus, but any effects that would alter this are resisted by Parma. The potential flaw in this case would be that a magus could shrink a boulder to pebble size and pitch it at the magus, knowing the Parma would effect it and remove the magic, turning it back into a boulder.
Perdo IS resisted, though it would be possible to "go around" the magic resistance by doing something like removing the ground from underneath the magus' feet or felling a tree so it falls on him. That would, of course, require finesse.
Rego IS NOT resisted, but things flung at magi must be aimed using Finesse. For example, a rock moving toward a magus would have any impetus stripped from it, but it would still have momentum, just not an acceleration. It would be aimed, but could also be blocked by ReTe magic.
This seems a very simple method, makes good use of the Finesse ability, and eliminates completely the "Pink Dot" loophole. Most importantly, it avoids confusion and halting the game to have a troupe discussion on rules permutations.
Of course, people will always exploit an opponent's defenses if they are able to. This is a natural human behavior. Personally speaking, I'd just like to see it require something beyond the spell I posted above in order to do it. Defenses DO have weaknesses by their nature, after all.
Of course, if you don't like it, I'll be happy to post my spell to turn an entire force of enemy soldiers and their gear entirely pink.