Fraction of blue galaxies as computed in different colours, based on the DS definition from Fig. 1 and colour–colour selection from Fig. 2. It is compared with observational data and with the original Butcher-Oemler curve in B − V, extrapolated to high z as f_bl = (z/ 2) − 0.015 (mid panel, dot-long-dashed line). The lower limit for selecting samples is given in terms of redshift-variable absolute M_B,M_V and M_K magnitudes (see text for details). The Poissonian error on f_bl is given by σ²(f_bl) = N_bl(N − N_bl) /N³ (see De Propris et al. 2004) and shown only for SIM cluster cores in the U − B plot as reference (upper panel).