Why do you wear red on pentecost?
Red is symbolic of two things: 1. The Holy Spirit (who appeared as tongues of flame on the disciples on Pentecost). 2. The Precious Blood of Jesus, which is received in the form of wine at Communion. Boys may wear a red sash at their First Holy Communion (so everyone can see it's their First Communion), but when they receive Communion on subsequent occasions, they just wear regular clothes.
Advent is purple and blue. Christmas and Christmas season is white or gold. Season after Epiphany is green. Lent is purple. Holy Week has red, white or gold and gray. Easter and Pentecost is celebrated with white or gold for Easter and red for Pentecost. Season after Pentecost is green. Other holy days and observations are red, white or gold.
They wear green for most of the year because it indicates that we are in ordinary time, meaning that we aren't in a significant celebration or time of preparation. There is also purple worn during Lent and Advent (pink during 3rd week of Advent), White worn during Easter and Christmas season, and red for Pentecost/ for the martyrs.
A priest wears red on Good Friday. Red is symbolic of warmth and blood. The liturgical color of red is used for Masses of the Holy Spirit, recalling the tongues of fire on Pentecost; on feasts commemorating the Passion of Christ; and on festivals of saints who were martyred. extracted from Modern Catholic Dictionary by John A. Hardon, S.J. Doubleday & Co., Inc. Garden City, NY 1980.