First, distinguish
Water disperses. An artist medium binds.
Water wets the finely ground leaf powder, helps to work it into a paste and controls the consistency of the application. It does not dissolve the material completely and does not form a binder of its own after drying.
A water-only test is useful for observing colour impression, grain, transparency and brush marks on a particular surface. Adhesion may be limited after drying, so this test is not automatically a permanently bound paint.
For work where adhesion or durability matters, choose an artist medium suited to the technique and surface and use it only according to the manufacturer's instructions. Always make a small test before starting the final work.