Colour-colour diagram used in the second selection step. The solid line is the isochrone computed from an evolutionary model with a dusty atmosphere (the AMES-Dusty model; the masses in M_⊙ for each z − K_s colour are shifted for clarity). The dashed lines show our selection area. We also show the theoretical colours of six galaxies as thin lines and the theoretical colours of red giants as thick lines. The six galaxies are two starbursts, one Sab, one Sbc, and two ellipticals of 5.5 and 15 Gyr, with redshifts from z = 0 to z = 2 in steps of 0.25 (evolution not considered). We assume that all red giants have a mass of 5 M_⊙, 0.5 < log g < 2.5 and 2000 K  <  T_eff < 6000 K.