Empirical colour vs. G−K_s calibrations for B−K_s, G−J, g−r and g−i colour indices. The dash-dotted lines correspond to our calibration at fixed metallicities (see legend). The colour of the points varies as a function of the metallicity. Diamonds correspond to the outliers removed at 3σ during the MCMC process. At the bottom of each calibration plot the residuals of the fit as a function of G−K_s and [Fe/H] are shown. All plots are scaled to the same G−K_s colour and metallicity intervals.