Electrical installation: how to label wires?
Heat-shrink labels all the way. They never fade or fall off. But you may not be able to use them if the cables already have connectors at either end.



You can get nice little labelling machines to write neatly on them.



Alternatively, try cable clips, which can be clipped on after the connector.



Another great one is these twist on cable labels.



There are literally dozens of solutions to this problem. If you want to google it, the key words are cable identification.