[α/Fe]  versus (v − y) − (b − u) for our 1498 calibrating stars. Bluer (redder) colours indicate hotter (cooler) stars, according to their (b − y), as shown in the top right box. Squares are synthetic colours computed from MARCS model fluxes at fixed  [α/Fe]  (as available from MARCS library) for selected values of (b − y) = 0.4,0.5,0.6 (from left to right) and  [Fe/H]  = 0.00 dex with  [α/Fe]  = 0:0.4 dex (continuous line)  [Fe/H]  =  −0.25 dex with  [α/Fe]  = 0:0.1:0.4 dex (dotted lines),  [Fe/H]  =  −0.50 dex with  [α/Fe]  = 0:0.2:0.4 dex (dashed lines),  [Fe/H]  =  −0.75 dex with  [α/Fe]  = 0:0.3:0.4 dex (dot-dashed lines),  [Fe/H]  =  −1.00 dex with  [α/Fe]  = 0:0.4 dex (triple-dot-dashed lines). Stars within 0.01 mag of the selected (b − y) interval are shown with open circles to highlight the trend. Grey dotted lines are fiducials built for those stars. Lower left panel is a zoom of the (b − y) = 0.4 data set for 2.2 ≤ (v − y) − (b − u) ≤ 2.8.