Photometric diagrams for 11 areas containing massive-star candidates (area label at top of each panel). The MS locus is the thick blue line; thin blue lines enclose the selected region (Sect. 5). Coloured circles mark sources at 2.2–3.2 kpc; grey dots are outside this range. Massive-star candidates are shown as blue circles (labels give ST). Potential low-mass stars are yellow; discarded sources are green.