Something with a different body shape is your best bet, Purple would probably be ok but every fish has a different personality and Purples can be nasty.... Yellow or Sailfin would likely be a safer bet.... but never a guarantee they wont fight.
I introduced a Purple, Blonde Naso and Sailfin into a 220g that already had a Yellow as the only tang for 3+ years
The Yellow and Purple went at it straight away and wouldn't stop.... everything I read said that it likely wouldn't stop until 1 was dead.
tried putting mirrors all over the tank. It distracted them for a few seconds at a time, lights went out with them still fighting. I decided to leave lights out for 72hrs and hope for the best. When it was time to turn the lights back on I thought for sure 1 would be dead, but to my surprise.... there they were, swimming together just grazing away!!