  Stellar population synthesis (SPS) models have long struggled to reproduce
observed optical through near-IR (NIR) spectral energy distributions (SED) of
massive quiescent galaxies. We revisit this issue using a novel approach that
combines the diagnostic power of full-spectrum fitting with recently updated
stellar spectral libraries. First, we perform full-spectrum fitting of
continuum-normalized stacked SDSS spectra in bins of velocity dispersion to
infer their stellar population properties, such as the elemental abundances and
age. Next, we use the resulting best-fit parameters to compute $ugriz$ colors,
which are then compared to observed colors of the same galaxies. With this
approach we are able to predict the $ugriz$ SEDs of low and high mass galaxies
at the $\lesssim 0.03$ mag level in nearly all cases. We find that the full
optical through NIR SEDs of quiescent galaxies can be reproduced only when the
spectrum is fit with a flexibility that is able to capture the behavior of the
entire optical absorption line spectrum. The models include variations in
individual elemental abundances, nebular emission lines, and the presence of
young stellar components. The successful prediction of the SED shape from
continuum-normalized spectra implies that the continuum information is largely
contained in the narrow absorption features. These results also imply that
attempts to model broadband photometry of quiescent systems will suffer from
potentially significant biases if the detailed abundance patterns are not taken
into account.
