I don't think that's as clear cut as most people on here will try to tell you. It's not based on how strong you are. I think the severity of your problem is more of a factor. It seems the original protocol has you stepping up from easier to harder as condition improves.
I started with the green. However, I've ordered a red one because my soreness has increased if anything. I think the green is limiting me from going through the full range of the exercise without substantial pain.
The reviews make the FlexBar sound like a miracle, instant cure. If that's what you're expecting and you have a more than very minor case of TE, you'll likely be disappointed. I remain optimistic that I'm on the right track with the FlexBar but only time will tell. TE is a bad thing and very difficult to really get rid of.