In the liturgical calendar, the color for each day corresponds to that day's main liturgical celebration, even though Optional Memorials (perhaps with a different color) might be chosen instead. The four main colors shown are:
White – Christmas Time and Easter Time; celebrations of the Lord (except of his Passion), of Mary, of the Angels, and of Saints who were not Martyrs; solemnities of the Most Holy Trinity (Sun. after Pentecost), of All Saints (Nov. 1), and of the Nativity of St. John the Baptist (June 24); feasts of the Conversion of St. Paul (Jan. 25), of the Chair of St. Peter (Feb. 22), and of St. John (Dec. 27); also permitted in the United States at Masses for the Dead
Red – Palm Sunday, Good Friday, and Pentecost Sunday; celebrations of the Lord's Passion, of Apostles, and of Evangelists; celebrations of Martyrs
Violet – Advent and Lent; Masses for the Dead
Other liturgical colors not represented on this calendar include rose (Third Sun. of Advent and Fourth Sun. of Lent), black (Masses for the Dead), and gold or silver (permitted in the United States for more solemn occasions).
See the full Liturgical Calendar for more information on all the liturgical celebrations available each day.
(Source: General Instruction of the Roman Missal, no. 346)