Colour-magnitude diagram Y − H versus K_s (based on G10) of sources detected in YHK_s in the environment of MRC 0156-252 (black dots). The r-YHK and b-YHK-selected galaxies are shown in red and blue circles respectively. Bigger symbols (with error bars) mark sources within 1 Mpc of the radio galaxy (at z = 2.02). Red sequence models at z = 2.02 for formation redshifts of 4 and 5 are shown by the solid lines (G10). Spectroscopically confirmed structure members (|z_spec − 2.02| < 0.015) are shown by blue squares (with error bars). The radio galaxy is the bright red source with K_s ~ 18.7. The gray box designates r-YHK galaxies with K_s < 22.5 and 1.8 < Y − H < 2.3 whose density was compared to fields surveys in Sect. 6.2. The FORS2 observations did not permit us to confirm (or deny) redshifts for any of the red sequence member candidates.