This paper shortly describes the development, method and validation of a memory colour based metric for the assessment of the colour quality of white light sources. The basic assumption of the metric is that the colour quality of a light source increases as object colours closely resemble what is expected. The similarity with an object's memory colour is described by a set of similarity functions derived by modelling the observer ratings of the colour appearance of a set of real familiar objects. The ratings were obtained in a series of visual experiments in which familiar objects were presented in many different colours to a group of observers. The observers were asked to rate the presented object colour with reference to what they thought the object look like in reality. Based on the psychophysical data of an experiment conducted by the authors, as well as visual data from several visual studies described in literature the memory colour metric was validated, with high statistical significance (p<0.0001), as a metric with a high correlation (r=0.88) with the visual appreciation of light sources. A comparison with 12 other metrics showed it was significantly better at assessing the colour quality of white light sources in terms of visual appreciation.