Keep your purple prisoners for ongoing t̶o̶r̶t̶u̶r̶e̶ debriefing and medical missions. Do thew prisoner exchange with your blues. Don't ever do the deliver mission. At least with exchange, you usually get a blue (sometimes purple) doff in return.
Originally Posted by voporak
So yes... you could get everything here for free.
Just like you can get a free steak dinner by collecting dropped pennies in the parking lot.