Decontamination process for the BSS and RGB samples. Panel (a): optical CMD for the stars sampled by the WFC3 observations within one half-mass radius from the cluster centre (grey dots). The blue circles mark the position of the BSSs selected using the near-UV box drawn in Fig. 3, and the blue contour enclosing these objects is the adopted BSS selection box in the optical CMD. Panel (b): optical CMD of the stars sampled by the ACS parallel observations (grey dots). The blue contour is the optical selection box defined in panel (a), and the blue circles marks all the stars included within it. Panel (c): optical CMD for the WFC3 stars sampled within one half-mass radius from the cluster centre (grey dots). The RGB selection is marked in black. Panel (d): optical CMD of the ACS stars sampled (grey dots), with the same selection box marked in panel (c). The black circles highlight the stars included within it.