Behaviour of the molecular index D_CO as a function of (J − K_s) (upper panels) and the atomic index Na I as a function of (J − H) (lower panels) at different metallicities and ages older than 2 Gyr, compared with galaxies from Mármol-Queraltó et al. (2009, MQ09). The models and the observations were convolved to a velocity dispersion of 350 km s^-1-before measuring indices. The error bars represent the typical uncertainties of the elliptical galaxy colours and indices.