Average colours (g − i) for the central parts (R <  0.5R_e, bottom panel), for R >  R_tr1 (middle panel), and for R >  R_tr2 (top panel) of the FDS ETGs located in the high- (red circles) and low-density region (blue circles) of the cluster as a function of the total stellar mass. The red and blue dashed lines in all the panels represent the least-square fits of the values for the central parts (R <  0.5R_e) of the ETGs in the high- and low-density regions of the cluster, respectively. The continuous red and blue lines in the middle panel are the least-square fits of the values for R >  R_tr1, while the dashed and the continuous red lines in the top panel are the least-square fits of the values for R <  0.5R_e and R >  R_tr1, respectively.