Grisaille (grĭ-zī', -zāl') (from French gris, "grey") is the term for paintings done entirely in monochrome. It is especially used to imitate a relief.
A grisaille could also be used as a first stage in an oil painting, or as a template for an engraver.
Among the churches with grisaille painting in Rome, the best examples are probably those found in Santi Vincenzo e Anastasio.