Colour–colour diagrams Y−H vs. H−K for all the fields : Top panels: control fields: GOODS1 (left), GOODS2 (middle) and CF2 (right). Bottom panels: HzRG fields: MRC 1017-220 (left) and MRC 0156-252 (right). We plot galaxies detected down to the 2σ detection limits in Y, H and K_s (black dots). r-YHK and b-YHK galaxies are designed by red and blue circles. Arrows indicate lower limits for sources detected in H and K_s but not in Y. The radio galaxies are marked by the yellow stars in the bottom panels. We also indicate with black open squares the two EROs found in the field of MRC 1017-220 (bottom left panel, Cimatti et al. 1999) and the two sources found within 5″ of MRC 0156-252 (bottom right panel) corresponding to objects B (the b-YHK galaxy) and C (the r-YHK galaxy) in Fig. 9.