There are quite a few different theories/explanations for the "meaning" of a red door. There are a few interpretations here, and I'll add that it's also said that, in the case of a church door, the red door often means that the church offers sanctuary. Originally, this means someone running from a police force or some such could run inside, and the police had to respect the sanctuary and could not arrest/etc. the person as long as they remained in the sanctuary, but in the more modern sense, churches will interpret the idea of sanctuary in a variety of different ways. Either way, it's still generally related to its roots of the right of asylum in a broad sense, at least when the red door is interpreted as meaning sanctuary. Just thought I'd share!

Someone needs to water their plants.

Belloc

Reminds me of Daenarys and her childhood "home". All she remembered was red doors, and a lemon tree outside of the window.