Intrinsic colors of our Quaia reference quasar sample, obtained after correcting the Pan-STARRS Survey DR1 grizy photometry (Chambers et al. 2016) for the Milky Way extinction according to the Schlafly & Finkbeiner (2011). The color of the points marks the distance of the quasars from the M 31 center ρ in units of ρ₂₅ isophotal radii. The distance between the bins was set to ensure these bins contain similar number of quasars. The solid lines connect the median colors within 0.5 dex wide redshift bins. The vertical error bars are the standard deviations of the colors of all quasars within each redshift bin, regardless of the distance from the M 31 center, and they were calculated as 1.48×MAD (median absolute deviation) to suppress the influence of outliers.