(u_ISL − r)₀ color difference with respect to the reference locus ZSL_o (top panel) and the ZSL (bottom panel) as a function of (g − i)₀ for the MS calibration stars with LAMOST information in pointing p_id = 00066. The colored scale in both panels shows the spectroscopic [Fe/H] from LAMOST. The dashed lines mark zero offset. The dotted line marks the median difference with respect to the ZSL. The derived metallicity offset is labeled in the bottom panel.