‘Grisaille’ is the term for scenes in illuminated manuscripts where the scenes are executed in shades of gray (‘gris’ in French) or brown. The backgrounds are often brightly colored or gilded. The effect is almost the same as looking at a statue or a carved frieze.
While this technique has been found in some English manuscripts, most of this style of painting is found in manuscripts from the continent.
How did It come about? Who thought of it? It seems to me, looking at the calm, almost monumental, scenes executed in this fashion, that the artist-scribes had to be thinking of the shimmer of light over polished marble. The touches of color, like the golden ray descending in the painting to the left, and the heavenly blue sky, lend a richness to the images.
The jewel-like decorations bordering the illustration of John the Baptist, right, would be lovely sandwiched between clear sheets of rock crystal, framed in gold, and hung from a chain. (Notice that he is wearing animal skins…)