I tried this when Jambin starting being a bit less keen to have a bath at nighttime. Voila - instant cooperation. He likes to choose what colour bath he has each night. Red is definitely his favourite. Sometimes he'll tell me he wants a green bath, then, after I've already put the food colouring in, changes his mind. I haven't been able to convince him yet that adding red to green is likely to result in a brown bath, not the desired red.
(these photos were obviously taken pre-haircut, a month or so ago)