I'm curious, too!
My parents have a bright red door, and when we were looking to repaint our house after purchase, lots of paint sample cards showed houses with bright red doors.
Incidentally, the color we changed it from was gray with a bright blue door.
I had read something about red doors being good luck in Feng Shui. I just looked it up and got this.
Q. "I've heard that painting my front door red will bring good luck. Is this true?"
A. If you'd like to bring new energy to your home, painting your front door a different color can help. Red is often recommended because in feng shui the color red is associated with success and prosperity, but that may not be the most appropriate choice for everyone. Choose a color that you like, that will be appropriate to the rest of the house exterior, and that supports your goals.
Black and dark blue are the colors associated with opportunities, social contacts, and career success.
Green is associated with wood energy, which is uplifting in nature. Lighter shades of green are associated with new growth and new beginnings; darker shades of green are associated with the energy of mature trees and forests. Both green and purple are associated with "fortunate blessings" which include (but are not limited to) financial prosperity.
Yellows and browns are associated with the stability of earth energy, but may not be the best choice for a front door, where you would like the chi to be more active.
White is associated with metal and creativity, but it will show dirt more clearly, so be sure to keep it clean.
It doesn't say anything about bright blue.