Red is typically used in connection with the blood of the martyrs, but Pentecost does not have that emphasis.
For more than a thousand years the Catholic Church has used various colors to denote feast days and liturgical seasons within the Church year. These colors have deep spiritual meanings and were chosen to fit the particular day that is being celebrated.
When it comes to Pentecost, the Roman Rite has chosen the color red. Typically this color is reserved for days celebrating martyrs who shed their blood for the Gospel or on feasts of Jesus’ passion such as Good Friday, when Jesus shed his own blood for our sake. In that context, the use of red on Pentecost does not make sense.
While Pentecost does not represent the blood of sacrifice, it does correspond to two other spiritual meanings that the Church has recognized. According to the Catholic Encyclopedia, red “is the language of fire and blood, indicate[ing] burning charity and the martyrs’ generous sacrifice.” With that in mind, red for Pentecost can be seen in reference to the flames of fire that came down upon the apostles, as well as the fire of “burning charity” that arose in them after that day.
In Christian art the Holy Spirit is most often represented by a dove, encircled by a blaze of fire. Additionally, many saints are depicted as having hearts on fire, symbolizing the fervor of their charity.
On the other hand, Orthodox Christians and many Eastern Catholics use the color green for Pentecost. This is done to emphasize the new creation that the breath of the Holy Spirit brings and the newness of life in the Spirit.
In the end, both colors used by Catholics emphasize different aspects of the Holy Spirit, who is our Advocate and Guide, given to us to “set the world on fire” with God’s love. For as Jesus said, “I came to cast fire upon the earth; and would that it were already kindled!” (Luke 12:49)