Red rum. Red rum. Red rum. Okay, now that we've officially creeped ourselves out, let's talk about Rahab's red cord. As we already know, the cord served as a signal to the Israelites to not attack Rahab and her family during the siege of Jericho. But why is it red?
The color red (or scarlet) is usually associated with sin and passion. Given that we know Rahab is a prostitute, the color of the cord might be a symbol of her profession, much like the red "A" on Hester Prym in The Scarlet Letter. However, the Bible does not view Rahab as a sinner. Here's an awkward math problem to help you understand:
Rahab's love and charity > Rahab's moneymaking ventures Red cord + Joshua's compassion = A living Rahab
Okay, that "math" problem doesn't really answer why the cord is red. Maybe this will.
Since the siege of Jericho is a "no survivors" kind of siege, the red cord hung out of Rahab's window is meant to symbolize the Passover, when God came over Egypt killing the first born children if there was no lamb's blood above the door. Just as God spared the faithful Israelites, so too do the Israelites spare the faithful Rahab.
So let's try some math again:
Red cord + Compassion (for Rahab) = symbol of Passover (God's mercy)