I can tell you what it looks like to me, having seen and analyzed many of Rubens' paintings, including some studies, sketches and drawings.
His designs were blocked in with the shadows first, most often in transparent browns, and then the transitions from the shadows to the middletones were addressed in opaque mixtures, then the middletones, also opaquely, and lastly the highlights, the most heavily applied passages. He used opaque greys to indicate turns slightly away from the light in the areas of primary light, i.e., half-shadows, in his undulating surfaces of flesh of light-skinned people. The chroma is highest in the middletone, and then begins diminishing as the form turns away from the light by the adding of greys to the wet middletones, and also as the form turns more toward the light by the adding of white and the color of the light source until at the highlight there is more of the color of the primary light source and white than there is of the body color. In this way, he indicates the form across the surfaces. Each form appears to have been executed wet-into-wet, at least from the edge of the shadow into the light, and perhaps all the way from shadow to highlight.
Where there is reflected light in shadows on flesh areas indicated, an opaque red-orange, possibly vermilion or a bright earth red, is used to represent the color of the flesh surface from which the light is reflected, registering on flesh of the same approximate color in the shadows. The chroma of the reflected color is increased when the color of the surface upon which it registers in shadow is the same color, and this is the effect Rubens was representing in these instances. What I have found works best in these situations is to paint the secondary light in while the shadow color is wet, rather than to add it on top after it is dry. A different optical effect is created when it is done wet-into-wet than when it is done in two layers. Wet-into-wet gives a more unified appearance, and a more solid illusion is created in that way.
The middletones in Rubens' flesh areas of light-completed subjects appear to be mixtures of lead white, perhaps lead-tin yellow, earth yellows, bone black or charcoal, and perhaps earth reds and/or vermilion. Red lakes such as madder or cochineal (carmine) may have been used in certain details, possibly in indicating where the lips meet, etc., but I would not go so far as to say that with certainty just on the basis of my memory of paintings I saw many years ago. What reads as blue in the flesh areas is probably not blue, but cool greys made from white and black, which appear to be blue in warmer surroundings.
I recommend you seek out Rubens' paintings in museums wherever you can find them, and spend a lot of time contemplating them, analyzing them, studying them, until you gain some insight into how they were done. Go alone, so no one will be distracting you from concentrating on the paintings for as long as you need to. The Old Masters have much to teach us, and it is there to be read in their paintings if we go to the trouble to study them closely enough. I recommend also studying the work of Jacob Jordaens, whose methods were similar to Rubens' in many ways. The museum in Brussels has many of his best paintings.
I hope that proves helpful.
Virgil Elliott
