event:unit enter region (your region)
condition:whatever....
action:Animation - unit vartex color (then put like 100% 50% 25% <-- something like that, its under animation.
Run trigger LeaveRegion <-- your second trigger.
2nd trigger: event: unit leaves region (your region)
condition: whatever....
action:Animation - unit vartex color (then put like 100% 100% 100% <-- unit normal color.