let students paint paper with watercolors, wet on wet, similar colors. Let dry and cut into triangles. Students pick triangles and glue down randomly, leaving white around each one. Maybe try with black or other color background?
On dry paper, paint the shape of the droplet using the same colors used to paint the petal or leaf. It should be slightly darker on the side that is nearest the light source. ~You can lighten the opposite side of the droplet by adding a tiny drop clear