The problem stems from the fact that negative ground has been the standard since late 1950's/early 1960's on vehicles and then tractors a few years later.
As a result we have let ourselves think that a battery charger output has a RED/HOT cable and a BLACK/GROUND cable. This thinking works for negative ground configuration but throws us into confusion on a positive ground tractor.
A battery charger knows and cares nothing about "hot" and "ground".
The charger has a RED POSITIVE cable and a BLACK NEGATIVE cable.
As expressed, Positive to Positive, Negative to Negative, forget about "ground".
Same rule applies to jumper cables.