The essay analyses the interest of Viollet-le-Duc in the relationship between decorative painting and architecture in French medieval buildings. This interest is documented by many of Viollet-le-Duc’s studies in which he tries to find the basic rules used by medieval painters to find the harmony of decorative painting. His purpose was to apply these rules in the restoration of medieval historic buildings. From the analyses of these studies, a new, modern interpretation of medieval conception and classification of colour emerges. In this sense, the studies of Prosper Mérimée, often quoted by Viollet-le-Duc both in the Dictionnaire raisonné de l’architecture française du XIe au XVIe siècle and in the Entretiens sur l’architecture, are also re...