The only places I ran multi-conductor wire is for avionics components like headphone jacks, since they originate and terminate in the same locations (back of the radio.) For most every other connection, the positive is run from a bus/fuse block or contactor, and the negative is to a grounding block in a different location, so single-conductor wire made the most sense. I just used white TEFZEL wire, with proper labeling color doesn't matter.
I used 20AWG for light loads (avionics components, magneto grounds, relay control), 18AWG for medium loads (12V socket/USB charger, master contactor control), 12AWG for heavier loads (connecting busses, starter button), 10AWG for my alternator circuit and 6AWG for the starter circuit. I used fuses appropriate to the capacity of each wire size and haven't blown a fuse yet.
The only harm in going oversized on wire is weight and cost (and space to some degree.) By far the majority of my runs are 18 or 20 AWG and all would have been fine by just pulling all 18AWG, but I saved a little weight and money by running 20AWG where appropriate.
Label your wires at both ends! I used a small-font printed slip of paper slid under clear heat-shrink a few inches from each end of the wire for my labeling.