Good luck! Water Fasting is much easier when you drink water only, otherwise your the trickle of taste for food will quickly turn into an uncontrollable torrent, that's what I find anyway.
Having said that, I think on my first fast I drank redbush tea for a day or so, because I had never done a fast before, so I know exactly where you are coming from. Then I went on to do 27 days. So if you must, drink it now rather than later. I am sure it wont harm. Green tea is healthy!