J − H,K_s − L′ colour–colour diagram of the central field of the Quintuplet cluster. The solid black line depicts the extinction vector (Nishiyama et al. 2009), and the red line is offset by 3 × the standard deviation of main-sequence cluster members without excess sources (green points). All L′ excess candidates are labelled (diamonds), with Wolf-Rayet candidates shown in blue (K_s< 12 mag), while fainter excess sources marked in red are candidates for circumstellar disc emission. Non-cluster members are shown in red, and black symbols depict sources with unknown membership status. The J − H,K_s − L′ colour–colour diagrams of the outer fields can be found in Appendix (B.1).