g′−r′ colour differences (defined as the colour of the 1–2R_e region minus the inner 0.5R_e region, see Eq. (16)) as a function of stellar mass (upper) and halo-centric distance (lower), split between Fornax main (blue) and Fornax group (orange). Δ(g′−r′) > 0 implies a redder outer region, whereas Δ(g′−r′) < 0 implies a bluer outer region. The solid coloured lines in the upper plot denote the median values calculated within bins of 1 dex in stellar mass, whereas those in the lower plot show the linear fits. The dotted line denotes Δ(g′−r′) = 0. The Spearman’s ρ and corresponding p-values for Fornax main and Fornax group are annotated in the plots. The KS-test for Δ(g′−r′) between Fornax main and Fornax group provided a p-value = 8.7 × 10⁻¹⁰.