Coefficients a, b from Eq. (1) as a function of the effective wavelength for each filter m, where m = B, g, V, G, r, i, I, z, Y, J.H, Ks. We note that the filter H was included here to better constrain the fit, even though it is not present in the analysis in the rest of the paper. The fit was performed for isochrones with combinations of age and metallicity for the sample clusters from Table 1, which are represented by different colours, mostly overlapping points. The line is a polynomial fit to all points and the shaded area represents ± 3σ uncertainties.