Hi Crazyleggedsigma!
I'm not a SigEp...obviously, but I do know the deal with the red doors, so I thought I would reply to your question!
It was a trend that started at the NY Alpha Chapter at Syracuse University in the 1920's and it's caught on - and I think all of them now have red doors.
Hope that clears things up!
Greek Love,
Casey
Kappa Kappa Gamma
ZA Chapter
"On the heart of each sister lies one key that binds us."