For colonoscopy prep day, a little peppermint tea (I found a tea bag at Starbucks that has peppermint, spearmint, and lemon verbena) seems nice. It isn't on any list of drinks I've seen, but I can't think of a reason it wouldn't be allowed - tea bag so no leaves, and it's not the wrong color. No caffeine.
This would just be in the morning way before all that other stuff later on.