Why Does the Pope Always Wear White?

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Coat of Arms of Pope St. Pius V (r. 1566-72), Santa Maria Sopra Minerva

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Why does the Pope always wear white? The custom is said to have started with Pope St. Pius V (r. 1566-72), a member of the Dominicans, who wanted to retain his Order's habit, which is white.

However, the wearing of white by popes predates Pius V by three centuries. A white cassock is first mentioned in a document dating back to the reign of Pope Gregory X (r. 1271-76).  

The coat of arms of Pius V is displayed on the facade of the Dominican church of Santa Maria sopra Minerva.  In addition to stating his title as Pontifex Maximus, it also refers to his membership of the Order of Preachers (ORDO PRAEDICATORUM), as the Dominicans are also called. 

Pope St Pius V, who was canonised in 1712, is buried in the Cappella Sistina, in the church of Santa Maria Maggiore. 
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• Related post: Papal Coats of Arms.