An important aspect to galaxy modeling is the the metallicity of a stellar population, generated by the production of heavy elements by supernova explosions.

The increase in heavy elements is referred to as chemical evolution and is the reason that halo stars are low in elements such as Mg and Fe, and disk stars are enriched.

This complicates galaxy modeling because stars that are weak in heavy elements (underabundent in astro terms) are bluer than metal-rich stars. This is due to a combination of line blanketening effects (blue photons shifted to red for high metallicities) and higher opacities (metal-rich means more sources of free electrons).