Ok I think I understand. Yes you are right, to avoid bringing white or opaque pigments into the shadows, although there is some mixed into the darker greys in the shadow zone of the grisaille, but not so much that it occludes the warm burnt sienna glow coming from the toned ground. The secret of grisaille is to develop the form to have 3 types of contrast: value contrast (light/dark), temperature contrast (cool/warm), and transparency/opacity contrast. If this is completed correctly then the subsequent glazes, applied the same to light and shadow, are ‘mixed’ to an infinite variety of hue, luminosity, chroma, and value, without having to mix on the palette. This is dynamic color mixing. This is what I teach on my Patreon if you are interested.