You didn't mention which country/continent, so I will assume US. I have worked in off-road engineering for >14yrs and have not come across any wiring color standards, SAE or ISO. Two of the companies I worked for did have their own internal standard, but it always gets compromised when you have to add a new widget or feature. So it works OK for your standard circuits (engine start, charging, lights, seat switch, etc.) but not for the other circuits.
I have worked on countless brands of machines and reviewed manuals from many others. CaseIH and Hyster/Yale use a type of ladder layout that works good for understanding the particular circuit and keeps all the circuit components on one page, but doesn't show where the component is in the vehicle. (grounds run along the bottom of the page, supplies on the top) Most use a compressed line drawing that spills over multiple pages that is hard to follow - till I get out my markers :-(. What I value most is an ACCURATE READABLE wiring diagram.(and many miss one and/or the other!) The second most helpful thing is circuit numbers on the wires. With both color and numbers there should be no confusion.
One story - I spend one whole day trying figure out why a track loader would not move. It was obviously something wrong with the safety interlock and I suspected a relay. There were 3 different versions of the wiring circuit depending on VIN#, and none of them matched so I had to trace the wires with my DMM. And to make matters worse they hid the damn relay under a bolt on kick panel but didn't show it in the manual! Not happy. Should have been a 20 minute job that my technician could have figured out.
ISZ