You need to make a trigger zone and put the following script as its "main" script.
NOTE: I have not yet tested the script but it should work. If it doesn't please let me know ASAP so i can fix it
NOTE: I don't think need to put the trigger zone on any specific location on the grid. I'm pretty sure that it work if you put it anywhere.
;Artificial Intelligence Script
;Header
desc = ambience=25
;Triggers
:state=0:ambience=25,ambiencered=25,ambienceblue=25,ambiencegreen=25
;End of Script
*Credit is appreciated but not necessary.