Top: C_mean (see Eq. (18)) is plotted as function of the metallicity. C_mean corresponds to the mean differences obtained with the different SBCRs (θ₀(i)) and the mean angular diameter obtained with the SBCR (F_V, V − K), θ₀(V, K), expressed as a percentage of θ₀(V, K). Bottom: Same but considering the amplitudes (see Eq. (19)). The confidence ellipse of the covariance (the ellipse enclose 95% of the points) is plotted in the figure, with the corresponding degree of correlation between the two parameters indicated in the legend. The vertical grey band corresponds to the metallicity of the stars in our sample used to calibrate the SBCRs.