Hmm, yeah there's no inverse action of "on collision", so you could use the closest thing, which is "is overlapping with". This tests if the two are actually overlapping, which isn't possible with a wall which I assume you've set to have a solid behaviour.
My solution: create a new object, called WallBuffer, which is dynamically spawned and resized to match every wall instance in the map, but is a few pixels wider. Test if the player "is/not overlapping with" this buffer (which is invisible, and isn't set to solid), to change your opacity to 50% and back.
A quick capx to illustrate my point: here
EDIT: Ah, beaten by Nimtrix. That solution is actually more elegant, setting opacity to 100% at every tick, which is obviously then counteracted by the 50% opacity event. Nice.GeometriX2013-01-31 15:22:00