[Thanks to Luke A. Chiniche Jr. for sharing on REALTORs® Round Table] 

The red front door has a lot of history and meaning. The Chinese consider red to be a lucky color. Many doors are painted red before the Chinese New Year and in Feng Sui, a red door symbolizes the mouth of the home allowing chi to be drawn to the house.
In America a red door was a sign of a home that was a safe stop for travelers. Catholic churches painted their doors red to remind people of Christ's sacrifice. Some believe red doors were used to represent safe houses along the Underground Railroad. Also, it is believed that people would paint their front doors red to indicate their home was paid off. 

Found these articles which basically confirm and expound (adding their own interesting red door pictures): https://www.debbie-debbiedoos.com/what-does-having-a-red-door-mean/  

                                                                                                                                                                                                                  https://www.humoroushomemaking.com/the-red-door-significance