Lustres are a kind of ceramics from the Mediterranean basin dating from Middle Age and Renaissance times that display astonishing colorimetric properties: their colour changes with respect to the direction under which they are observed. These properties are related with the structure containing nanoparticles of noble metal produced by the potters. In fact, various effects play a role in the aspect of these ceramics: plasmon absorption, interferences and diffusion. Interferences occur in the specular direction, as is commonly seen, but also in diffuse directions. In this paper, a model able to simulate these interferential effects in the diffuse directions is developed in order to understand the optical properties of the lustres. The model integrates all the phenomena mentioned above and leads to good results in reproducing the colour effects observed. The results obtained here give some clues in order to understand the reasons that may have pushed the potters to improve and control the difficult process they were using at that time to produce those ceramics.