Which wire ?

[BrianG]

The only wires where polarity really matters is the power input. If you switch those around wrong, you will have a non-operable ESC.

But the motor wires don't matter. Like was said, if the motor spins the wrong way, swap any two of the motor wires. Or, if the wire was laid out so it is aesthetically pleasing and the motor spins the wrong way, hook the ESC to a computer and use Castle Link to reverse motor direction. Either way, doesn't matter.

The only type of system where motor wire polarity matters is sensored systems.