Colour map of the relative error (ε_rel, 90%) obtained after fitting the SBF for mock galaxies of different brightness and SBF magnitudes ($m,\overline{m}$). The fitting was performed for three centred annular masks, with fixed r₁ = 4 px, then r₂ = R_eff for panel (A), r₂ = R_eff/2 (B), and r₂ = R_eff/3 (C). The black circled cross signals the reference galaxy presented in Sect. 2.1 ($m=9.493,\overline{m}=30.64$).