It’s been my experience that if you copy the file /etc/xdg/compton.conf into your ~/.config directory, you can then modify that file to suit your own needs.
At that point, compton will use the file in ~/.config/compton.config
This answers your question if you don’t wish to modify the system file in /etc/xtg (which may not actually be advisable).
That ought to be the file that is being used. You might want to see if the same file exists in just ~/ it might be .compton.conf
I just rechecked mine - it is definatly ~/.config/compton.conf
The file in ~/.config will be used by default unless it is not found. Then it will use the other in /etc/blah
Are you also restarting compton after you make the change?
I normally do a killall compton then run compton -b OR a simple logout and login should do also.