  We combine Spitzer $3.6\mu$ observations of a sample of disk galaxies
spanning over 10 magnitudes in luminosity with optical luminosities and colors
to test population synthesis prescriptions for computing stellar mass. Many
commonly employed models fail to provide self-consistent results: the stellar
mass estimated from the luminosity in one band can differ grossly from that of
another band for the same galaxy. Independent models agree closely in the
optical ($V$-band), but diverge at longer wavelengths. This effect is
particularly pronounced in recent models with substantial contributions from
TP-AGB stars. We provide revised color--mass-to-light ratio relations that
yield self-consistent stellar masses when applied to real galaxies. The $B-V$
color is a good indicator of the mass-to-light ratio. Some additional
information is provided by $V-I$, but neither it nor $J-K_s$ are particularly
useful for constraining the mass-to-light ratio on their own. In the
near-infrared, the mass-to-light ratio depends weakly on color, with typical
values of $0.6\; \mathrm{M}_{\odot}/\mathrm{L}_{\odot}$ in the $K_s$-band and
$0.47\; \mathrm{M}_{\odot}/\mathrm{L}_{\odot}$ at $3.6\mu$.

Astrophysics of Galaxies