Peter Green, if I remember the story correctly, was messing about with his neck humbucker, took it apart, then put it back together with the magnet upside down. He thus flipped the polarity of the pickup. He attempted to fix this by putting the pickup in the wrong way round, with the slug coils nearest the neck, but this didn't fix the problem. If he had connected the wires back to front as well, which would have been difficult with a vintage PAF with a single core "hot" plus a braided sheath "ground", he would have created a reverse wired reverse polarity (RWRP) pickup, and fixed the problem. But then he wouldn't have created that sound.
If you want to recreate the Peter Green sound, you can take the pickup apart, flip the magnet by turning it through 90 degrees once (and once only), putting it back together and wiring it in normally. Alternatively, you can just flip the black and green wire connections. Electromagnetically speaking either should do, but not both together. Do both and youve created a RWRP humbucker. Remember the bare, if there is one, always goes to ground though. For the last part, put the pickup in the wrong way round. Some say having the neck pickup slug poles nearer the neck subtely changes the neck pickup tone and is the final touch.