[W2 − W3]−[W1 − W2] and [W3 − W4]−[W2 − W3] CCDs. Red triangles are the sources from the template sample in Table 1. Small black triangles are selected sources (see main text). Small dots are non-selected sources; to avoid cluttering stricter SN criteria were applied for them to be plotted (see main text). Other symbols indicate sequences of increasing MLRs (see main text). Blue colours represent C-star models while orange colours represent O-star models for (T_eff, T_inn) combinations of (2600, 1000), (3300, 800), and (4000, 400 K) in diamonds, five-pointed stars, and open plus signs, respectively.