The Method of Tempera Painting). But even so, why put the green there in the first place? It is obviously intended to give a greenish tinge to the skin, even before the colour changed over time.
When painting icons I was encouraged to use red and green washes but it was an intuitive process and I wasn't sure exactly what principles were supposed to be guiding me (I probably hadn't been paying attention in class!).
It wasn't until I went to study portrait painting in Florence that I felt I understood what was going on here. I was taught that we see the deepest shadows are red or red brown and the half-tones are a green grey. The colour is obtained by mixing ivory black, yellow ochre and white. Ivory black has a touch of blue in it and so creates a grey-green when mixed with the yellow and white. The colour produced matches almost exactly the colour of the veins that you see, under the surface of the skin. So if you look at your wrists you will see this colour in the visible veins under your skin there.
What the icon painters and gothic artists are doing is just what the portrait painters were trying to do: match the colour scheme of their paintings with what they see in nature. It shows that even in the highly stylised forms so much is based upon observation of nature.
Above: a study I made under direction in which the deep shadow lines are red (see the line above the upper eyelid and defining the nose for example); below: a mosaic by a master from Istanbul in which the artist has defined the deep shadow lines in a red brown colour, and the half tones are in earth green. This is handled superbly and has a natural appearance.
Above: a pastel portrait completed while I was in Florence the deep shades are red-brown while the half-tone skin colour is grey green; below: a portrait of 18th century society beauty Lady Hamilton by master George Romney in which he is very skilfully handling the transition from green to red as the shadow gets deeper. And below that is a self-portrait by another the 18th century English master, Sir Joshua Reynolds. Romney and Reynolds are part of the British school that can be traced back to Sir Anthony Van Dyck who was, in turn the star student of Rubens.