Stars with spectroscopy on the PCMs of NGC 0104, NGC 288, NGC 362, and NGC 1851. In the left panels, the photometric classification of MPs is adopted; and in the middle panels, the population assignment is based on spectroscopy. Our PCMs are indicated with small grey dots. In the right panels, stars considered mismatches in classification are superimposed to the Na−O anti-correlation (as filled symbols). Solid lines in each GC indicate the limit [Na/Fe]_min + 0.3 dex (with a range ±0.07 dex, dashed lines) used to separate FG and SG stars. The colour-coding is as in Fig. 5.