The effect of the metallicity parameter is to produce redder colours by increasing the amount of metals contained in the galaxy. Because the same colour effect can be reproduced changing the age or the reddening of the galaxy itself, the metallicity can be regarded as a secondary parameter in the construction of the reference SEDs. Therefore, we decided to use prevalently the synthesis models with a fixed metallicity, set equal to the solar metallicity: $Z=Z_\odot\simeq 0.02$, with $Z$ the mass fraction of heavy elements in the interstellar gas.

We explored the effect of considering different extreme metallicities provided by GISSEL98 and template SEDs with metallicity evolution. To this goal we used the approximation of the closed-box model, considering the ejection of heavy elements from each generation of stars, without assuming the instantaneous recycling hypothesis. The tests we carried out (Bolzonella et al. 2000) confirm that the metallicity can be regarded as a secondary parameter. In any case, the files containing SEDs with evolving metallicities can be found in the tar files zphot_big_endian_evol_MiSc.tar.gz and zphot_little_endian_evol_MiSc.tar.gz.

micol bolzonella