Comparison of the galaxy distribution in the i − K_s versus J − K_s diagram for the circular region of 1′-radius containing the five Herschel/SPIRE sources and a region external to it (i.e. the “field”). An excess (ϵ, see Eq. (2)) of galaxies with i − K_s ~ 2.3 and J − K_s ~ 0.8 is detected at the location of PHz G95.5−61.6 with respect to the field (see text for further details). For visualisation purposes, the map has been smoothed with a Gaussian beam of two pixels. Tracks that describe the colours for three typical SED templates of local galaxies, observed from z = 0 to z = 4, are over-plotted. For each track, filled black circles indicate the loci at a redshift step of 0.5, with added labels at redshifts 1, 2, and 3.