SBF magnitude as a function of color. Left, middle and right panels show the three different fields analyzed in this work, with the same order as in Fig. 1). The color bar represents the velocity of the sources, v_CMB. The various symbol shapes represent the assigned quality flag for the SBF measurement: circles for galaxies classified as q1, triangles for q2, and “X” for q3. Different sizes distinguish bright from dwarf galaxies. The dashed black lines represent the calibration formula from Cantiello et al. (2024) shifted as discussed in the text. The shaded gray area is the mean cosmic scatter from the relation σ_cos, as estimated in Cantiello et al. (2024).