Should be added to your player bullet / shooting events.
You don't need instance variables because they're different objects.
Have your global variable set to 1,2 or 3.
In your shooting logic, something like :
bullet on collision with green, if gv=1 destroy, else do nothing
bullet on collision with red, if gv=2 destroy, else do nothing
bullet on collision with blue, if gv=3 destroy, else do nothing
I might not be understanding the game correctly through from the description, if so let's see a video or the game file.