Metallicity of individual OCs of the HRS sample determined as a free parameter in the procedure. The grey colour denotes this work; the green colour shows the predictions of the ANN of Cavallo et al. (2024). Figure layout is the same as in Fig. 4: X-axis shows the [Fe/H] from high-resolution spectroscopy, the bottom plot shows the difference between the values, and the black line indicates 1:1 relation on the top plot and difference of zero in the bottom plot. Clusters with three or fewer red giant branch (RGB) stars are marked with an X symbol. The number in the bottom left corner indicates the median difference and RMSD.