Catholic Fancy Dress?
At St Luke's there is a tradition of the clergy wearing a Biretta, a black coloured headwear. The biretta was originally academic dress, reserved to graduates of higher degrees of universities. In the 16th century, however, their use was allowed to all clergy whether or not they held higher degrees. Thus they became distinctive clerical wear. They are in no way illegal in the Church of England, even if the wearing one seems somewhat eccentric.
They can be worn in liturgical celebrations, that is church services. There are rules about when to put the biretta on and when to take it off. Thus, you may have seen the clergy birettas putting their hats on and taking them off. The tradition is that the biretta is taken off and replaced upon any utterance of the Holy Name, ie Jesus Christ. It is accepted practice to remove the biretta and keep it off after the third utterance of the Holy Name.
When Our Lady (Mary) is mentioned the biretta is touched with the right hand but not removed.
The pom pom on the top is usually black. It does vary however: for canons, red; for bishops, purple. In the case of bishops the entire biretta is usually purple.
You may have noticed clergy wearing over their black cassocks what looks like an extended cumperbund or cloth girdle - this is a cincture.
The cloth cincture is black and if the priest is a member of the Society of the Holy Cross (SSC) is piped in red.
Clergy who serve in a bishop's 'household' (for example as secretary or chaplain) may wear a Roman purple cincture. Any Vicar General, Archdeacon, Dean and Canons may have red buttons and braiding on their cassocks and capes and wear a Roman purple cincture.
Not fancy dress at all
Wearing clerical dress is itself a form of ministry. Not only do those who wear it bear witness to the fact that they are citizens of the kingdom of Christ but they also present themselves to the world as people who can be approached when people are in need of the love of Christ. At ordination clergy are set apart and their very beings aligned with Christ in a special way precisely so that he might touch the world through us and use us as a catalyst for his grace. It is therefore logical that they should look different and be easily identifiable as people who stand in persona Christi.