It's a very artistic metaphor for life. Whenever someone looks at the door they are reminded that life is unfinished. Change for the better will still come, but in an unknown time. Just like the time that this door will be panted is unknown. You can take a peek into it's future, but can't quite see it. It represents all of us. Not quite at our full potential. Just like the color of the unpainted door doesn't quite blend in with the colors of the room, we do not quite fit into our place in society until we are finished. This is the door to adventure! This is the door to life!
In all seriousness, they probably ran out of paint, but I just thought I'd have some fun with it.