A near-infrared J − H vs. H − K_S colour − colour diagram for sources in the studied field with high quality (q = 3) AKARI/IRC fluxes. Values are plotted in the 2MASS photometric system; the main sequence, dwarf and giant loci, as well as the CTTS loci (Meyer et al. 1997), are drawn along with reddening vectors. The colour bar and the size of the circles indicate the value of spectral slope (α) calculated over the K to 9 μm or K to 18 μm range of the SED (see Sect. 4.2 for an explanation).