the different vestments worn by the priest 

 - Gifford summarizes:

White for Easter and Christmas, feasts of the Trinity, for Christ, the Virgin Mary, angels and saints who were not martyrs; red for Pentecost, feasts of the Cross and martyrs; purple for Advent, Lent, penitential occasions.  Green, gold, rose are also used.  Black is the color of priestly robes on Good Friday.