I was pondering this today and wondering why white is not used. Luckily my 1962 Missal came up with an answer:
Red signifies the fire of love towards God and is therefore the liturgical colour for Whitsuntide, the Feast of the Holy Ghost, the God of Love;....
It did strike me how visceral red is. How the Holy Spirit needs to be indwelling within our bodies for God's love to be manifest. The Holy Spirit doesn't hover over the Church, He needs to burrow into each and everyone of us and find a welcome home. Penetcost surely then is the feast of God's sublime gift to us and the red is our Passion united to Christ's and ignited by the flame of the Holy Spirit.