In this case you can use a piece of printable clear plastic, which your local art supply store should stock. Place the plastic over your painting and paint in, with watercolor, the object you are considering, onto the plastic. If this looks good then go ahead and paint it on the original - otherwise try something else or just leave it as it is.
This gives you a lot of freedom to try various things without damaging your painting.
Happy painting,
Joe Cartwright
