A monochrome is a work painted in a single colour, but the term is often used more loosely to describe works in which a single colour predominates. In such pictures it is the subtle variation of tone which creates the desired effects.
There was a school of landscape painting in Haarlem in the early 17th century which painted 'monochrome' landscapes. The school included Salomon van Ruysdael, Jan van Goyen and Pieter de Molijn.
See the entry for Grisaille.