However, I think that old masters added it to eliminate the heavy turpentine smell. But be careful, use these last 4 colors very sparingly. In the second layer all details are made excluding the texture. The picture must look as if its objects were lit with moonlight - olive cold gray color. Colors are applied thickly, half a tone higher, shadows are very transparent, half a tone lower.
Observation is a very important aspect of learning to paint in the classical style.
To master the technique, it is best to watch a master paint for long periods of time. In the the contemporary system of teaching, the student sits down right off with a handful of brushes and a palette of paint, and starts slapping it on the canvas, while the instructor looks on and makes comments or suggestions now and then. It is therefore, very important for those wishing to seriously pursue classical technique to just watch for a while. Drawing on paper. A Few words about Flemish technique.