if you have a sample of the colour (ie. on part of the car) certain bodyshops will have a useful little device called a photospectrometer which will scan the paint surface and pull up a list of matching colours on a computer, you can also input search parameters like "fiat" and "non-metallic" for instance and it will find the fiat non-metallic colour nearest matching the scan. maybe get them to spray a test card out of the paint you have to scan or just scan the cars paintwork if possible!
that will get you the right code, then just a case of getting it put in aerosol can...