You don't need OpenIV for this. In order to edit these files, you need to find handling.dat and vehicles.ide in "common>data" and open them with Notepad.
You can find an explained handling line guide here: http://www.gtagaming.com/forums/showthread.php?t=118800
To add the "+livery" to vehicles, open vehicles.ide and find the vehicle you want, go to the "flags" section (right at the end of the line) and add "+livery" on the end.
Then, go to you "trainer.ini", press F3, search "livery" and change "vehiclelivery" to "=0". This allows you to change liveries in the trainer menu.