its illegal for the lab tech to smell the urine, not to mention thats a little sick. Even if they get "a whiff" of it, they cant turn it down unless there is something completely obvious with it. It could be any color and any odor. So, unless they physically see you putting a substitute in there, or its too dilute and cant get a sample with it, they cant turn it down. So, the only thing you need to do is get it at body temperature.