I Googled "Red Door Symbolism" and found this:
A red door is a significant symbol in many countries. In China, some say a door is painted red before the New Year to invite good luck and happiness, while in Ireland, it’s done to ward off ghosts and evil spirits.
The scarlet entry also has deep religious undertones. In Catholicism, a red door represents that the blood of Christ has been smeared on it and that the area beyond the door is holy and sacred. For Protestants, and especially Lutherans, the red door harkens back to the time of Martin Luther, who posted his 95 Theses on the red doors of the Castle Church in Wittenberg, Germany—the crimson color symbolizes the church as part of the Reformation.
And for homeowners, a red door announces that the house is paid for, free and clear.