Selection and properties of 168 NISP-only sources in Abell 2390 and Abell 2764. Sources are divided into Lyman-break galaxy (LBG) candidates and extremely red sources (ERSs). Panel A: I_E − Y_E versus J_E − H_E colour-colour diagram highlighting the significance of the VIS non-detection (1.5σ lower limits) and general colours of the sources. Panel B: Y_E − J_E versus J_E − H_E colour-colour diagram separating the two samples. Also shown is a reddening vector as well as tracks of z = 6–9 LBGs with different mean stellar ages (grey) and of z = 1–3 maximally old quiescent galaxies (maroon). Colours of observed M-, L-, and T-type dwarf stars are measured from the SpeX Prism Library (Burgasser et al. 2004). Typical colour uncertainties are shown in the upper two panels, where all I_E − Y_E colours are lower limits.