I've now actually tried this and can't make it work. I've added the code you suggested to the Action pane for the container I'm using for the map, but when I add an adornment to the map the assignment of Color or Prototype respectively doesn't happen.
To trouble-shoot, I simplified the OnAdd action to $Color="red", and then created a note and an adornment in the map. The action works on the note but fails to work on the adornment
So it seems that OnAdd actions don't apply to adornments created in maps?