A Monsignor in my diocese - here in Scotland - uses a Mantelletta and a Biretta with a Violet Pom. He isn't the highest rank of Monsignor, but continues to wear it in regards to Tradition. The Violet Pom is because he is also a Canon of the Cathedral Chapter. He's a great guy, and always celebrates Mass with such piety and dignity; with respect for tradition and reverence for Our Lord. Deo Gratias!
Perhaps the mantelletta is part of the vesture of being a canon of the cathedral chapter.