Top panels: two SEDs of quiescent galaxies. SED in magenta represents a population of 1 Gyr redshifted to z = 3 (i.e., z_form ≈ 6), while the cyan line shows the SED of a population of 0.5 Gyr redshifted to z = 4 (i.e., again z_form ≈ 6). Filters transmission curves used in COSMOS field and in the tracks evaluations are also shown. Bottom panels: JK_s[3.6][4.5] and HK_s[3.6] color–color diagrams with different evolutionary tracks both for star-forming galaxies and passive galaxies. Tracks representing star-forming galaxies are shown in cyan, blue and black with A_V = 0, A_V = 3, and A_V = 5 respectively. Three tracks for quiescent galaxies (with exponentially declining star formation history τ = 0.1 Gyr and solar metallicity) are shown in magenta, orange and red with different extinction values (A_V = 0, A_V = 1, and A_V = 2 respectively). Gray shaded areas represent the selection region for quiescent galaxies at 2.5 ≲ z ≲ 4 in the left panel and at 3 ≲ z ≲ 4.5 in the right panel. The numbers near the tracks of the same color represent the redshift. A vector corresponding to a magnitude extinction of A_V = 1 using Calzetti’s law is also shown.