'65 Dart temp gauge / wiring questions

Don't know for sure on your '65, but most all the temp sensor wires I remember have been violet.
Download the service manual for your car, it has all the wiring diagrams in it. Free at Mymopar.com.
Might have to get the Plymouth version, but the wiring is basically the same.
Service Manuals – MyMopar
And, you want to put the aftermarket gauge sensor before the thermostat, not after it.