If I understand correctly, you want to have rain that falls, and when it hits a solid make a raindrop mark on the solid? If that's the case I would create another sprite for what the water looks like sitting on the ground, then when the raindrop collides with whatever you want to activate it, destroy that raindrop and create the water mark sprite, then you could keep it there indefinitely, destroy it a bit after its created, or you could use the fade behavior (That would probably look the best)