The norms for the US are actually set at one of two levels:
- For the Romans, by the USCCB.
- For the Eastern Catholics, per the rules of their particular church if there are any, or the same as the Romans if not.
The USCCB guidelines I’ve read are that priests should wear a black clerical collar shirt with black trousers or a black cassock when engaged in any public non-liturgical ministry, and that’s based upon (and matches) a norm recommended by one of the committees in Rome. According to Noonan, The Roman norm also recommends the Biretta, and permits priests a black zuchetto, but that’s seldom used, and in the US, the Biretta is seldom seen except for bishops.
Bishops are supposed to wear a cassock under the alb for liturgical use as well, and all clerics may.
Transitional Deacons and Seminarians, and Subdeacons, may wear the same. Permanent deacons are more vague; the Roman guideline is that married permanent deacons should also wear the clericals when engaged in ministries, but left the decision to the local bishop.
The Byzantine Catholic Churches often add a pectoral cross to the Riassa/Raisson (Cassock) for priests, and sometimes for deacons. I’ve also seen the pectoral cross worn with the Clericals. Bishops of the Byzantine Rite also wear their englopia (a worn icon) with both cassock and clerical suit.
Some Byzantine Catholic Dioceses have had married clerics in blue or gray Raisson (cassock), others have them in black. Those which do might also have them wear matching clerical shirts. One such diocese changed that with a change in bishop…
At least one Roman diocese had their permanent deacons in slate-gray clerical shirts; I don’t know if that diocese still does. Another does not permit the married permanent deacons to wear the clerical suit unless employed full time in a clerical task; the Cassock is permitted but not mandated, but the alb and stole is used instead most of the time; any priest or deacon engaged in non-clerical employment is to wear civil attire with some symbol of Christian Witness.
According to James-Charles Noonan, Jr., in The Church Visible, the cerical suit should always be black for Roman diocesan clerics; a papal indult is present for white in tropical climes. Noonan further makes note that, if the clerical black vest is used, it may be worn over any color shirt, but the cuffs should be white, and with the jacket, the shirt should not be visible.
A Note: some orders of religious who are not clerics wear the clerical suit and/or cassock as part of their habit; a few use colors other than black. Blue is used for a few as a sign of their devotion to Mary.
As a further note: Priests and deacons in religious orders are subject to the rules for the habit of their order.
I’ve seen Dominican priests do their public non-liturgical ministries in Habit, in Clerical Suit, and in civil attire over a clerical collar (especially when working with the homeless); I’ve seen the Cassock for them only in liturgical ministries, and always with surplice.