In Feng Shui, a red door symbolizes the mouth of the home. By painting our door red (or any bright colour that stands out) chi (positive energy) is drawn to the house. It is the entry point in which abundance and opportunities find us.
The Chinese consider red to be the lucky or sacred colour. Many doors are painted with a fresh coat of red just before Chinese New Year to invite good luck and happiness.
In Ireland, a red door is supposed to ward off ghosts and evil spirits.
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.
Also, according to the Old Testament, the Jewish slaves in Egypt smeared their doors with lambs blood as a sign that the required sacrifice had already been made, and those homes would be passed over by the Angel of Vengence.
Supposedly red doors were used as part of the Underground Railroad, and homes with red doors were "safe houses".
Albert Einstein painted his door red because he couldn't recognize his house without it.
And for homeowners, a red door announces that the house is paid for, free and clear.