  We explore the relation between the dynamical mass-to-light ratio ($M/L$) and
rest-frame color of massive quiescent galaxies out to z~2. We use a galaxy
sample with measured stellar velocity dispersions in combination with Hubble
Space Telescope and ground-based multi-band photometry. Our sample spans a
large range in $\log M_{dyn}/L_{g}$ (of 1.6~dex) and $\log M_{dyn}/L_{K}$ (of
1.3~dex). There is a strong, approximately linear correlation between the $M/L$
for different wavebands and rest-frame color. The root-mean-scatter scatter in
$\log~M_{dyn}/L$ residuals implies that it is possible to estimate the $M/L$
with an accuracy of ~0.25 dex from a single rest-frame optical color. Stellar
population synthesis (SPS) models with a Salpeter stellar initial mass function
(IMF) can not simultaneously match $M_{dyn}/L_{g}$ vs. $(g-z)_{rest-frame}$ and
$M_{dyn}/L_{K}$ vs. $(g-K)_{rest-frame}$. By changing the slope of the IMF we
are still unable to explain the M/L of the bluest and reddest galaxies. We find
that an IMF with a slope between $\alpha=2.35$ and $\alpha=1.35$ provides the
best match. We also explore a broken IMF with a Salpeter slope at
$M<1M_{\odot}$ and $M>4M_{\odot}$ and a slope $\alpha$ in the intermediate
region. The data favor a slope of $\alpha=1.35$ over $\alpha=2.35$.
Nonetheless, our results show that variations between different SPS models are
comparable to the IMF variations. In our analysis we assume that the variation
in $M/L$ and color is driven by differences in age, and that other
contributions (e.g., metallicity evolution, dark matter) are small. These
assumptions may be an important source of uncertainty as galaxies evolve in
more complex ways.
