Stellar mass-to-light ratio as a function of rest-frame u − g colour, for three composite stellar population models obtained with GALAXEV (Bruzual & Charlot 2003). Each model has a fixed metallicity and an exponentially declining star formation history with a decay time of 1 Gyr. Each curve was obtained by varying the time since the initial burst. The grey lines connect points of fixed time since the initial burst, as indicated by the labels. The model corresponding to a metallicity Z = 0.05 is used in Sect. 6.1 to assign stellar masses to galaxies, given their observed colour.