Since the discovery of glassmaking, the colouration caused by transition elements, either brought by impure raw materials or intentionally developed, has always been on of the most attractive properties of glasses. This probably culminated during the middle age with the intense use of coloured glasses. Still now, many glassy products take advantage of glass colouration, including containers and tableware, art and design or functional glasses. Transition metal ions constitute the most important source of glass colouring agents. Colouration varies, for a given transition element, as a function of chemical and physical parameters such as glass composition or melting/fining conditions. At the same time, the electronic transitions responsible for light selective absorption and glass colouration provide unique information about the local structure and chemical bonding of glasses. This presentation aims to review optical absorption data at the light of complementary information provided by a broad range of experimental and numerical 5-fold coordination, distribution of site geometry, sensitivity to the chemical bond, medium-rate organisation, heterogeneous spatial distribution. Some of these structural characteristics are inherited from the peculiar dynamics of silicate melts and may show a significant modification as a function of temperature. As transition elements can be connected to the various structural subsets of glasses, they are useful colour indicates of the complex structure of these materials. Vice versa, using a better knowledge of the structural behaviour of transition elements, the variation of colours may be rationalised as function of glass composition and melting conditions.
