Distribution of all bona fide stars, selected according to Eq. (1), in the J − Ks vs. z′ − Ks colour–colour diagram, where 3′′-aperture photometry is used. This distribution is consistent with the loci occupied by spectrally classified stars with SDSS and 2MASS photometry (cf. Fig. 1 in Galametz et al. 2009). As shown by Galametz et al., stars with a spectral type later than K5 populate the region above the short-dashed line and tend to exhibit z′ − Ks > 0. Conversely, stars with a spectral type earlier than K5 populate the region blueward of the horizontal short-dashed line and define a linear colour–colour relation (solid line) with extensions towards very red z′ − Ks colours.