Behaviour of T_eff derived from IRFM by González Hernández & Bonifacio (2009) as a function of the colour for dwarf and giant stars (upper and lower panels, respectively). The stars are grouped according to their metallicity: [Fe/H] ≤–2.5 dex (blue points),–2.5 <[Fe/H] ≤–1.5 (green points), –1.5 <[Fe/H] ≤–0.5 (red points), [Fe/H] >–0.5 dex (black points). The solid lines are the theoretical colour–T_effrelation calculated with [Fe/H]=–3.0 dex (blue line), –2.0 dex (red line), –1.0 dex (red line), +0.0 dex (black line).