These are the red doors of Saint Mark's Episcopal Church in Philadelphia. There's no rule requiring red doors for Episcopal churches, but it's not uncommon to see them.
Reasons abound. Some say it was a sign in the Middle Ages that a person who was evading the law could find refuge inside. That the church was outside the reach of civil authority. Some say red is the Pentecost color. Pray that inside we'd find people who are spirit-filled with the love of God - God's mercy and justice. Or red doors were a sign of an Anglo-Catholic Church (altars, Virgin Mary, crosses, images, smells and bells), while Evangelical Churches, where the Word alone was preached, had gray or brown doors. That's kind of interesting.
Or maybe, the color red is a take off from Exodus 12:7, where Moses is instructed that on the night of the Passover, when Israel would be freed from Egyptian slavery, that a lamb was to be slaughtered and its blood put on the door posts to mark the Israelite houses and over which the angel would then pass-over, keeping the children safe. That strikes me as quite plausible.
Now we're the children who pass under or through the blood stained doors. But the ancient lamb's blood is now Christ's blood.
Blood is the stuff of life. But I'd suggest that it isn't only the Sunday church-goers, passing through the doors, who are kept safe from sin's death-claim, but we all are - all of humankind. The red doors might be a flashy, wordless way of telling folks: We're universally saved; come on in and hear about it. Come on in and celebrate what's true for and about all of us. Christ's gifts aren't parcelled out in small bits for some and not for others.
Remember the Cana wedding story? Jesus makes huge amounts of wine to keep the love-feast going. The whole village was there! It would be a ridiculous image to think of Jesus standing, warden-like, next to the clay jars filled with his amazing vintaged wine, ladle in hand, asking people to prove they qualify, deserve or somehow measure up before filling their mugs. The only qualifier is, "I'm thirsty and want to get in on the fun."
I like how in this wintry scene up top, the dawn sun seems to be lighting up the Calvary image over the doors. Jesus wide-armed! Christianity is supposed to illumine what's true about God and us.