Chronological and stylistic studies of Roman wall paintings and mosaics were based for a long time only on optical direct analysis and current analogue or digital photographic reproduction. The aims of fresco research today remain the same as in the past — i.e. pigment types and sources, painting subjects and styles, application methods, and the relationship between paintings and the function of their locations. Modern technologies, however, have now been developed which can finally improve our knowledge about ancient decorative taste and workshops. New methodologies such as highly accurate colour reproduction of paintings and mosaics, digital reconstruction of fragmentary decorations, or chemical analyses of pigments or plasters are increasingly being used to fill important research gaps. Some of these techniques are illustrated in this article as solutions to obstacles in the archaeological documentation of an unpublished first-century ad painting (Late Flavian period) on the Caelian Hill in Rome. With relation to Fourth Style decorations, the problematic question as to whether there was an intentional ratio between the component parts of the fresco is also discussed, linking for the first time the Vitruvian architectural concept of Commodulatio (meaning ‘with symmetry/proportion’) to wall paintings.