Just did mine the other day. Can't remember what the colour code is, but if you have a voltmeter you can find out very quickly.
If not:
- stick the xfer box in neutral with the handbrake on
- stick the main box in reverse
- touch the side of the bulb to an earthing point (the body)
- touch each wire in turn to the other terminal of the bulb and see which one makes it light up
Don't touch either of the wires directly to earth, of course, cos you'll blow the fuse.
David