Why are a Catholic bishop's Coat of Arms green and not purple?

Question

Why are a Catholic bishop's Coat of Arms green and not purple?

The Canadian Conference of Bishops tell us that the galero and tassels on the Coat of Arms for a bishop are green. Cardinals' galero and tassels are red.

What does a Bishop’s coat of arms look like?

The coat of arms is the emblem of the Bishop, which is used on letterheads, documents and other official publications. It consists of a shield bearing symbols representing the person for whom it was conceived. Around it, elements indicate his dignity, rank, title, jurisdiction and more. A scroll or banner on which is inscribed a motto or guiding principle usually sits beneath the shield.

The shield of every Bishop is unique. The symbols featured relate to many influences in his life: these may include references to people, communities, places and events that have had an impact on his spiritual leadership.

The crest and supporters of the coat of arms are common to all Bishops. A green galero (ecclesiastical hat) sitting upon a gold processional cross form the crest. A number of tassels on either side, determined by rank, make up the supporters (Bishops have six, Archbishops 10 and Cardinals 15). On a Cardinal’s coat of arms, the galero and tassels are red. - Canadian Conference of Catholic Bishops

Coat of Arms of Bishop Zarama the sixth bishop of the Diocese of Raleigh, North Carolina.

The silk cord for the pectoral cross of an archbishop or bishop is green and gold and a cardinal is red and gold. The pope wears a gold silk cord for his pectoral cross.

In choir dress—that is, when he wears a cassock, rochet and mozzetta—the pectoral cross is usually suspended by a cord of silk. This cord is green and gold for an archbishop or a bishop, and red and gold for a cardinal and gold for the pope. An abbot makes use of a black and gold silk cord while an abbess and canon would use a black silk cord. Formerly, protonotaries apostolic wore a pectoral cross on a purple silk cord when celebrating in pontificals. - Pectoral cross (Wikipedia)

Why are these insignias in green and not purple? After all bishops and archbishops don purple cassocks.

Answer 1

The reason why a Catholic bishop's Coat of Arms are green dwells in the fact that green was the original color worn by bishops and archbishops was green, not purple.

The shield is the green ecclesiastical hat called a “gallero” with twenty tassels pendant on both sides. This broad brimmed hat, once worn in cavalcades, is no longer used but remains as a heraldic emblem. The original color worn by bishops and archbishops was green, not purple. This “episcopal color” is retained in heraldry. These external ornaments are those used for a prelate with the rank of Archbishop according to the Instruction of the Holy See, Ut Sive, of March, 1969. - Armorial Bearings of the Most Reverend Thomas Rodi, DD Ninth Bishop and Second Metropolitan Archbishop of Mobile, Alabama

The original color for the cassocks of bishop and archbishops was green. In the 16th century it was changed to purple.

For most of Church history green was the color for bishops. This color is still seen on the traditional coat of arms that each bishop chooses when elected. However, in the 16th century the color was switched to “amaranth red,” which is named so in reference to the color of the amaranth flower. In practicality it more resembles the color fuschia.

Since it is a color similar to purple, it holds a symbolic value that points to the task of the bishop to govern his local diocese. - How to tell the difference between a monsignor, a bishop and a cardinal

Answer 2

Purple is the color of the Bishop's clothes only, and of the livery-garments at his ecclesiastical household. The true “episcopal color"—the one to be used for decoration purposes— is not purple, but green.

Taken from the footnote in Costume of Prelates of the Catholic Church by J.A.Nainfa