Examples of SDSS colour–colour evolution for a mixed population. This is composed of an old 10 Gyr underlying cluster plus a young burst (τ < 20 Myr and clear Hα emission). Different coloured lines represent a different mass contribution of the old population to the young burst (F, being mass-old/mass-young). Synthetic colours with and without the emission lines contribution are plotted as solid and dashed lines, respectively. Two metallicity cases have been chosen as examples: top panels use Z = 0.0001 and Z = 0.0004 for old and young population, respectively, while bottom panels use Z = 0.004 and Z = 0.008, as labelled in the plots.