  Isochrones for ages > 4 Gyr and metallicities in the range -2.5 < [Fe/H] <
+0.3 that take the diffusion of helium and recent advances in stellar physics
into account are compared with observations in the Johnson-Cousins BV(RI)_C
photometric system for several open and globular star clusters. The adopted
color-Teff relations include those which we have derived from the latest MARCS
model atmospheres and empirical transformations for dwarf and subgiant stars
given by Casagrande et al (2010, A&A, 512, 54; CRMBA). Those reported by
VandenBerg & Clem (2003, AJ, 126, 778) have also been considered, mainly to
resolve some outstanding questions concerning them. Remarkably, when the
subdwarfs in the CRMBA data set that have sigma_pi/pi < 0.15 are superimposed
on a set of 12 Gyr isochrones spanning a wide range in [Fe/H], the inferred
metallicities and effective temperatures agree, in the mean, with those given
by CRMBA to within +/- 0.05 dex and +/- 10 K, respectively. Thus the hot Teff
scale derived by CRMBA is nearly identical with that predicted by stellar
models and consequently, there is excellent consistency between theory and
observations on the H-R diagram and the different color-magnitude diagrams
considered in this investigation. To obtain similar consistency, the colors
obtained from the MARCS and VandenBerg & Clem B-V vs. Teff relations for
metal-poor dwarf stars should be adjusted to the red by 0.02-0.03 mag. In
general, isochrones that employ the CRMBA transformations provide reasonably
good fits to our BV(RI)_C photometry for main-sequence stars in the globular
clusters 47 Tuc, M3, M5, M92 and NGC 1851 - but not the cluster giants (when
adopting the synthetic MARCS colors). We speculate that differences between the
actual heavy-element mixtures and those assumed in the theoretical models may
be the primary cause of this difficulty.
