I'd have preferred to see the units, and the accompanying diagrams, before committing myself to an answer but I'd assume that a 'common line' electrically bonds the metal casings of the two components. So I'd expect to see a terminal on each component which is attached to the casing and to run a cable between the two. There might already be a wire attached to one of the two terminals, in which case, irrespective of the colour, that's the wire you need to use. Alternatively, you might be required to provide your own cable (of whatever colour you like).
Chris