Radial profiles for the g′−r′ images and the two models obtained in our dust-free colour calculation process. The red profile shows the median colour value of the original annuli. The green line shows the model, which follows the requirement that the colour cannot get redder with increasing radius (see text). The blue profile shows the final model, which is flat before 0.5 R_eff, and after 3 R_eff. The dotted vertical lines indicate 1 R_eff and 3 R_eff, respectively.