Comparison between the photometric and the spectroscopic g − r colours for galaxies observed in this work (empty triangles) and in Gavazzi et al. (2004) (filled triangles). The solid line gives the 1:1 relationship and the dotted lines the 1 sigma uncertainty. The cross indicates a typical error bar of 0.14 mag on the colour, corresponding to  ∼ 10% uncertainty in each band.