Glazing with perceived colors on monochromatic complementary under-paintings

In my winter semester classes, my students learn to glaze with perceived colors over complementary monochromatic underpaintings. This is a classical technique that is most familiar to oil or acrylic painting, but can be used to add dramatic colors to any medium, including watercolor.
Below are some examples of what my students did this semester!

Here, my student painted green for the apple and then painted red over it. Next they painted Violet for the pear, and glazed with yellow. The last one is blue for the pumpkin, with orange as the glaze.

This is one of my demos that I painted, to show the technique to my students. I painted the apple in violet, first. Then I glazed over it in yellow. 

My students had a blast with this lesson! I use this lesson to reinforce their learning of color theory, light and values, color mixing, and more. In addition to setting the values, letting some of the underpainting show through the glaze layer adds a dramatic pop of complementary color to paintings.