Through computed synthetic spectra for individual stars in the Mg2 index region where different values of [Mg/Fe] were considered, and observed features Fe5270 and Fe5335 in a sample of 80 stars, we derived relations Mg2 and <Fe> = f(Teff, log g, [Fe/H], [Mg/Fe]). These relations were used to compute integrated indices for single aged populations, where number counts in colour-magnitude diagrams were used. A multi-population model was then built through a convolution of the single stellar population indices with a metallicity and luminosity distribution for elliptical galaxies, where a chemical evolution model includes enrichment by type II and I supernovae. A grid of 80 multi-population models was computed, by varying the efficiency of star formation rate. In Figure 1 are shown the results for [Mg/Fe] = 0.0, 0.3 and 0.6 compared to observational data by Worthey et al. (1992). An average value of [Mg/Fe] = 0.3 is found for the sample ellipticals.