u − i color of the sample as a function of the projected distance from the Virgo cluster center (distance from M 87) in units of the cluster virial radius (R_vir = 1.55 Mpc; Ferrarese et al. 2012). The UDGs and the diffuse galaxy subsamples are marked with the filled red and open blue circles, respectively. The black star symbols are the sources detected in Hα (see Sect. 5.5). The solid black line and the red-shaded region give the linear regression best fit and the 3σ scatter, respectively, obtained using the linmix Python package (Kelly 2007). Sources with only an upper or lower limit in the color are excluded from the plot.