The origins of 'pink chit' are from the Pennisula war or 1809-12 or so. It was used by the officer of the day to give permission for troops to fall out on the march to relieve themselves. Any troop found out of the ranks without the chit signed by the OOD were liable to arrest and a good dose of field punishment. To stop false chits being made the OOD was given expensive pink paper to write these permissions on. This papaer was deemed to expensive for ordinary troops to buy. First known use was in Crawford's light division and is explained in the original unit standing orders for use on the march. Or so I've read somewhere long ago.