  In integrated-light, some color-color diagrams that use optical and
near-infrared photometry show surprisingly orthogonal grids as age and
metallicity are varied, and they are coming into common usage for estimating
the average age and metallicity of spiral galaxies. In this paper we
reconstruct these composite grids using simple stellar population models from
several different groups convolved with some plausible functional forms of star
formation histories at fixed metallicity. We find that the youngest populations
present (t<2 Gyr) dominate the light, and because of their presence the
age-metallicity degeneracy can be partially broken with broad-band colors,
unlike older populations. The scatter among simple stellar population models by
different authors is, however, large at ages t<2 Gyr. The dominant
uncertainties in stellar population models arise from convective core overshoot
assumptions and the treatment of the thermally pulsing asymptotic giant branch
phase and helium abundance may play a significant role at higher metallicities.
Real spiral galaxies are unlikely to have smooth, exponential star formation
histories, and burstiness will cause a partial reversion to the single-burst
case, which has even larger model-to-model scatter. Finally, it is emphasized
that the current composite stellar population models need some implementation
of chemical enrichment histories for the proper analysis of the observational
data.
