The CIELAB coordinate has demonstrated as useful for description of the colour characteristics of decolourised flint glass. The amounts of cobalt and erbium oxides necessary for sufficient decolourising of additions of coloured cullet can be calculated from the appearant linear dependence of CIE a* and b* coordinates to the added amounts of cobalt and erbium oxides to the batch. For the sucess of such a calculation, the colour point of the added cullet must be determined.