How to properly label power wires
For industrial applications:


PE for protective earth. Green / yellow for wiring.
0V for 24 V supply common is generally good because its meaning is clear. White with blue stripe is normal in Europe.
+24V is usually split by fuses, circuit breakers or electronic fault protection units so additional wire numbering is required. +24-1, +24-2, etc., would make it very clear. Blue wiring is common in Europe.


If the power must be isolated I would be inclined then the argument could be made for 24V+ and 24V- (with signs after the voltage)
. This should still be adequately clear and does not imply a &pm;24 V power system.