The CIEL*a*b* coordinate system has been demonstrated as useful for describing the colour characteristics of decolourised flint glass. The amounts of cobalt and erbium oxides necessary for sufficient decolourising of additions of recycled coloured cullet can be calculated from the apparent linear dependence of the CIE a * and b* coordinates on the amounts of cobalt and erbium oxides added to the batch. For the success of such a calculation the colour point of the added cullet must be determined.