While my reply doesn’t answer the OP, I thought I could provide some background to the papal attire. Depending on the source, the papal soutane/cassock had not always been white. Some historians point to the election of Innocent V in 1276 as the first Bishop of Rome to adopt the white attire. Other historians say it occurred much later, in 1566 with the election of Pius V. Interestingly, both men had been Dominican friars prior to their elections as the Bishop of Rome, and as the habit of the Dominicans is white, they chose to wear the habit of their religious order.
With respect to the attire of Eastern Orthodox/Catholic Bishops, there is a similar history. With the Triumph of Orthodoxy in 843 under Empress Theodora, the monks were seen to be more faithful to the Orthodox faith than the secular clergy who we’re usually more sympathetic to the iconoclasm of previous emperors. As a result, monks were appointed to the episcopacy, and these bishops continued to wear their (black) monastic habit. Even Eastern Orthodox/Catholic bishops who were never monks have adopted the black monastic habit as episcopal attire.
Bob, it was Metropolitan Jonah of the OCA who wore the full white habit, but only during the Paschal season. outside of the Paschal season, he resumed wearing the black exorason with white kamilavka.