A - Thanks for the question. There are several good reasons why priests wear black.
Reason #1 - "Clerics" are a kind of uniform for clergy. As in certain secular professions a uniform can help easily identify someone. Priests want to be easily identified as clergy, not so they stand out in a crowd and gather attention for their own sake, but because they should be a sign of Jesus to the world.
In the early Church there was no specific clothing for the ordained clergy. But, by
By the middle ages, canon law proscribed clerical garb to be worn and started to regulate it. It even had penalties for those who did not. There are no longer any penalties associated with not wearing clerical garb.
Thus, clerical clothing has developed down through the ages to today. Canon law presently states:
Clerics are to wear suitable ecclesiastical garb according to the norms issued by the conference of bishops and according to legitimate local customs - Canon 284.Here is what the USCCB says in a "complimentary legislation" on Canon Law about it:
In liturgical rites, clerics shall wear the vesture prescribed in the proper liturgical books. Outside liturgical functions, a black suit and Roman collar are the usual attire for priests. The use of the cassock is at the discretion of the cleric.So, in ordinary situations, a priest should wear clerics. But, it is not required at all times.
In the case of religious clerics, the determinations of their proper institutes or societies are to be observed with regard to wearing the religious habit.
Reason #2 - Clerics symbolizes something, as do most practices in our Church.
The black represents a priest dying to self as well as simplicity/poverty. Every time he puts on his clerics, he should remember that he does not belong to himself, but his bride, the Church. It also symbolizes simplicity and giving up the comforts, honors, and privileges of the world.
The white Roman collar you see priests wearing symbolizes obedience to God and the Church. This comes from the tradition of a slave having a ring put around their necks and priests choose to give their lives to Christ as his "slaves". It also represents the marriage "ring" of being we to the Church. The white also symbolizes the resurrection of Christ.
In other countries, especially in hot/tropical ones, you will find that the colors might be reversed and white might be the color of preference. It makes sense in hot climates not to make someone wear black all the time. Also, other colors for Monsignors, Bishops, Archbishops, Cardinals, and the Pope help others recognize their positions in the Church. They also each have a meaning behind them.
I hope this helps.