Wire color standards for a reversible DC motor
As the motor is reversible, I'd be inclined to use two wires of the same colour. This helps emphasise that you can't make any assumptions about them.
One thing I've learnt in years of engineering is that a colour scheme that's obvious to one person is anything but to another, unless it's mandated by some standard like mains wiring, where black is ground on the European side of the pond, and hot on the US side.