  This paper provides a detailed comparison of the differences in parameters
derived for a star cluster from its color-magnitude diagrams depending on the
filters and models used. We examine the consistency and reliability of fitting
three widely-used stellar evolution models to fifteen combinations of optical
and near-IR photometry for the old open cluster NGC 188. The optical filter
response curves match those of the theoretical systems and are thus not the
source of fit inconsistencies. NGC 188 is ideally suited to the present study
thanks to a wide variety of high-quality photometry and available proper
motions and radial velocities which enable us to remove non-cluster members and
many binaries. Our Bayesian fitting technique yields inferred values of age,
metallicity, distance modulus, and absorption as a function of the photometric
band combinations and stellar models. We show that the historically-favored
three band combinations of UBV and VRI can be meaningfully inconsistent with
each other and with longer baseline datasets such as UBVRIJHKs. Differences
among model sets can also be substantial. For instance, fitting Yi et al.
(2001) and Dotter et al. (2008) models to UBVRIJHKs photometry for NGC 188
yields the following cluster parameters: age={5.78+ 0.03, 6.45+-0.04} Gyr,
[Fe/H]={+0.125+-0.003, -0.077+-0.003} dex, m-M={11.441+-0.007, 11.525+-0.005}
mag, and Av={0.162+-0.003, 0.236+-0.003} mag, respectively. Within the formal
fitting errors, these two fits are substantially and statistically different.
Such differences amongst fits using different filters and models are a
cautionary tale regarding our current ability to fit star cluster
color-magnitude diagrams. Additional modeling of this kind, with more models
and star clusters, and future GAIA parallaxes are critical for isolating and
quantifying the most relevant uncertainties in stellar evolutionary models.
