It is customary, especially in Episcopal Churches, to paint the front doors red. The reason for this is obscure, but there are several proposed explanations. The most whimsical of these explanations is that, once upon a time, painting the doors of a building red signified that the mortgage had been paid in full.
In England during the Middle Ages, the red door was a symbol of sanctuary. If one were being pursued, say by the sheriff, or local gentry, and this person reached the church door, he/she would be safe from harm. The Church was not subject to civil law, and at this time, no one would dare commit a violent act on hallowed ground. The red door served both as a warning and a beacon. Once inside, those safely within could explain to the priest what happened, with the hope of justice being served in their favor.
Red doors also remind us of the Passover in Exodus. God commanded the Hebrews to mark their doors with sacrificial lamb's blood to protect them from his judgment on the firstborn in the land of Egypt.
For Christians, the sacrificial lamb's blood symbolizes the blood of Jesus "poured out for many for the forgiveness of sins" (Matthew 26:28). Red also represents the blood of the martyrs calling us to lives of self-sacrificial love, discipleship, and faithful witness to the good news of Jesus Christ.
Finally, red represents the fire of the Holy Spirit. When we enter the church, we place our lives in the protection of the gift-giving Spirit who births the church and us as its members, and who comforts, strengthens, challenges, refines, and transforms us. Every Sunday that we are about to hear the Scriptures, inspired by the Holy Spirit, read and proclaimed.