Colour–temperature relation of the MSTO stars in the working sample (colour-coded according to metallicity) and the isochrone models (blue points on a straight line), for the original derived AMBRE effective temperatures (left panel) and after the offset correction in T_eff (right panel). The linear Theil-Sen estimator method (red line) was applied to the stellar parameter values. The green lines delimiter the lower and upper bound of the 95% confidence interval of the Theil-Sen linear regression method. Middle panel: observed linear T_eff–offset (ΔT_eff = T_eff, MODEL − T_{eff, OBSERVATION}) for different metallicity values.