Start with bok choy or celery from the grocery store. Chop stalks 2 to 3 inches from bottom of bok choy or celery. Blot off moisture and let dry for an hour. Pour dark blue crafts paint onto paper plate. Use bok choy or celery end as a stamp: Press cut end into paint, then onto canvas in random pattern. Let dry. Repeat with lighter blue color, overlapping as desired.