If I bought a wiring harness described as a 'reproduction' and it didn't match the original colors, I'd send it back for a refund.
Using such a harness means you can't use the manufacturer's wiring diagram and will confuse the hell out of anyone who has to work on the car in future.
I always try to use what's already there.
In my experience, Fiat wiring harnesses give very little trouble.
Providing no-one has messed with it, all that's usually needed is cleaning/replacing a few terminals that are corroded due to exposure to mud/salt/moisture and ensuring that all connections are tight, especially at the fusebox, ignition switch.
This is a lot easier, quicker and CHEAPER that installing a replacement harness.
Bear in mind, fitting a new harness won't necessarily cure earth(ground) problems, these will still have to be located and corrected.....
AL.