  The temperatures and colors of stellar models are much less secure than
predicted luminosities; consequently, it is important to ensure that the models
which are used to interpret data for stellar populations satisfy existing
observational constraints on these properties. At the present time, the
available Teff constraints do not appear to pose a problem for evolutionary
calculations. However, the evidence seems compelling that adjustments of the
color transformations from model atmospheres are needed to achieve consistency
with observations of cool stars. Stellar models that are normalized to the Sun
and transformed to the observed plane using recent semi-empirical color-Teff
relations are able to provide superb fits to the M67 and Hyades color-magnitude
diagrams. As highlighted in the following examination of Population II
subdwarfs and several well-observed globular clusters (M3, M5, M68, M92, and 47
Tuc), it is much harder to constrain the Teff and color scales at low
metallicities than at [Fe/H] near 0.0. It is not clear that significant
improvements to current semi-empirical color-Teff relations for metal-deficient
stars will be possible until much tigher observational constraints become
available. Brief comparisons of the evolutionary tracks computed by different
workers are also included in this investigation.
