Hi Richest, it's nice to see you again :) I remember in my Theology class at University, the teacher once told us that the red doors are a traditional sign marking sanctuary. In olden days, soldiers were not allowed to cross red doors to arrest criminals or to kill other soldiers. Modern churches carry on threw tradition. As modern Christians, you're supposed to remember that the red doors mark sanctuary inside the church from the pressures of the outside world. Once you're inside the red doors, you're to give up whatever troubles you have and focus on Jesus. Hope that helps.
My Lutheran college has red outside doors for every building. My house also has a red front door. Everyone tells me it means "welcome". It's a just a traditional style. There are other meanings like the blood of Christ and whatnot.