The monochrome style of stained glass using gray or black paint is called grisaille.
The word comes from the French gris, meaning gray. In stained-glass work, grisaille generally describes largely monochromatic glass painting, often using gray tones to create outlines, shading, folds, facial features, and architectural detail. The painted surface is then fired so the pigment bonds to the glass.
Medieval and later glaziers used grisaille both on its own and alongside colored glass. It could make a window visually quieter, provide detailed figures, or imitate the effect of stone sculpture. Some grisaille panels include touches of yellow stain or other color, so the term does not always mean that every part is strictly gray.
The word is also used for monochrome painting in other media. In a stained-glass context, it refers specifically to the painted, gray-toned treatment of glass.