Make the colors represent the number of days until derailment, not distance from center of road.
I just started a goal with the week of flatness and my graph is blue, even though I'm nine days away from derailing! Also, I went on vacation a few months ago and put a few of my goals to zero while I was gone, and the colors for one goal told me I was going to derail even though I had a week and a half to derailment!
I think the "yellow brick road" analogy is okay, but it's just an analogy and adds an extra layer of interpretation in this and other cases (like the 0 safe day thing meaning that you've still got more than a day until derailment from your goal) that doesn't buy anything. Distance to the middle of the road is less useful than distance to derailment.
I really like the red=problem today, orange=problem tomorrow, blue=problem day after tomorrow, green=good for a few days (one weekend if it's Friday) that you get most of the time, but any time the future slope of the goal changes, the graph colors become useless.
Will Everett commented
Or, if you want to go nuts, allow the user to specify what the colors mean...