Red team operations can be effectively performed in close cooperation with the defensive (blue) team - this format differs in some key points from the classical redteaming or penetration testing. Organizational faults or wrong understanding of what is going on can decrease the overall effectiveness of the operation dramatically for both sides.
Looking from the side of red team, discussing, what purple teaming really is, how it should be carried out to be really effective and what are the main constraints on this way.