Look at the green shadow in this painting. this was achieved using a technique called under-painting.
In art, an under-painting is an initial layer of paint applied to a ground, which serves as a base for subsequent layers of paint and is paint that is intended to be painted over in layers. A complementary color under-painting was thought to be more useful by artists and this technique was often used in the Gothic and Renaissance period. The colors of the under-painting are often a muddy/earth like complementary color.
The green under the flesh makes an optically illusion that mingled with the subsequent over-painting it is giving the skin a glow. The over painting can not completely cover the under-painting some of the under-color must show through. The complementary under-painting for green nature should to bring life to the paintings. Under green we paint a earth red color.
In these pictures underneath you can see the flesh is a bit too green, over the years the over-painting pigment has faded so what we actually see is too much of the under-paint thus revealing the secrets these old masters used to bring life to their subjects.
Simone Martini and Lippo Memmi in the Uffizi Gallery, Florence.
When I saw these old paintings in real life I was stunned. The technique they use is tempera and gold on a wooden panel. This is technique is amazing. It shows how much they understood the importance of color and used it to the maximum. Look closely at the faces and you can see the green under-painting that brings the faces to life.