Left panel:  [Fe/H]  versus m₁ for our 1498 calibrating stars in different (b − y) ranges represented by different symbols (median values as indicated in the labels). The size of the symbols increases with higher values of c₁. Dotted and continuous lines represent Eq. (2) at these median values for the 10th and 90th percentiles of the c₁ distribution. Central panel: same as left panel, but for Eq. (3), which applies only to cool stars. Right panel: theoretical  [Fe/H]  versus m₁ relation when using synthetic colours from the “MARCS-standard” library and log g instead of c₁ (see text for discussion).