This paper shows the results of the application of S-CIELAB colour metric to digital images of the fragments coming from the S. Mathew’s fresco, in the S. Francis Upper Church in Assisi, broken during the earthquake of September 1997. S-CIELAB, a spatial extension to the CIELAB colour representation, is a colour space whose metrics closely reproduce, through Euclidean norm, the colour distances perceived by a human observer. S-CIELAB accounts for the effects due to the spatial distribution of colours into the image. It is based on the application of a suitable spatial filtering to the colour image data, aimed to simulate the spatial blurring measured on the human visual system. In the system for aided virtual recomposition of fragmented frescos we are developing, the interaction between the operators and the huge collection of fragments is grounded on a ‘query-by-example’ paradigm. On the base of a set of images of fragments provided as examples, the system browses through the whole collection and retrieves all fragments similar in terms of their colour and texture contents. The application of S-CIELAB colour metric allows the system to ascribe to each fragment its pattern-colour characteristics according to the visual perception of the human operators. The close approximation of human perception allows a meaningful co-operation, increasing the system’s efficiency in the retrieval of fragments and reducing the workload of the restorer.