Bruzual & Charlot (2003) model predictions of different stellar populations in a Y−H vs. H−K colour − colour diagram (see text for details on the models). The dashed line accounts for the two-colour selection designed to isolate galaxies at z > 1.6. The horizontal line (Y−H > 1.5) shows the single-colour selection to separate red passively evolving galaxies (r-YHK galaxies) from star forming ones (b-YHK galaxies). We assume an exponential declining star formation history with τ = 0.1, 0.3, 0.5, 0.7, 1 and 5 Gyr (see legend). Top: for increasing redshifts (z = 0 to z = 3, each 0.5 bin marked by coloured points) and a constant age of 2 Gyr, the stars corresponding to z = 1.77. The black arrow indicates the reddening vector E(B−V) = 0.2 as parameterized by Cardelli et al. (1989). Bottom: for various population ages (t = 1.5, 2, 2.5, 2.75 Gyr, coloured points with Y−H getting redder with ages) at z = 1.77 and z = 2.02.