Q. 1. Is blue a liturgical color? If not, can it be used instead or violet or purple?
A. 1. It is not permitted to substitute blue for purple or violet as a liturgical color.
Quoting from the "Committee on Divine Worship" of the "United States Conference of Catholic Bishops," Chapter VI, it states:
# 346. "As to the color of sacred vestments, the traditional usage is to be retained: namely,
White is used in the Offices and Masses during the Easter and Christmas seasons; also on celebrations of the Lord other than of his Passion, of the Blessed Virgin Mary, of the Holy Angels, and of Saints who were not Martyrs; on the Solemnities of All Saints (1 November) and of the Nativity of Saint John the Baptist (24 June); and on the Feasts of Saint John the Evangelist (27 December), of the Chair of Saint Peter (22 February), and of the Conversion of Saint Paul (25 January).
Red is used on Palm Sunday of the Lord's Passion and on Good Friday, on Pentecost Sunday, on celebrations of the Lord's Passion, on the feasts of the Apostles and Evangelists, and on celebrations of Martyr Saints.
Green is used in the Offices and Masses of Ordinary Time.
Violet or purple is used in Advent and of Lent. It may also be worn in Offices and Masses for the Dead (cf. below).
Besides violet, white or black vestments may be worn at funeral services and at other Offices and Masses for the Dead in the Dioceses of the United States of America.
Rose may be used, where it is the practice, on Gaudete Sunday (Third Sunday of Advent) and on Laetare Sunday (Fourth Sunday of Lent).
On more solemn days, sacred vestments may be used that are festive, that is, more precious, even if not of the color of the day.
Gold or silver colored vestments may be worn on more solemn occasions in the dioceses of the United States of America."
# 347. "Ritual Masses are celebrated in their proper color, in white, or in a festive color; Masses for Various Needs, on the other hand, are celebrated in the color proper to the day or the season or in violet if they are of a penitential character, for example, no. 31 (in Time of War or Conflict), no. 33 (in Time of Famine), or no. 38 (for the Forgiveness of Sins); Votive Masses are celebrated in the color suited to the Mass itself or even in the color proper to the day or the season."