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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 physicsinto account are compared with observations in the Johnson-Cousins BV(RI)_Cphotometric system for several open and globular star clusters. The adoptedcolor-Teff relations include those which we have derived from the latest MARCSmodel atmospheres and empirical transformations for dwarf and subgiant starsgiven by Casagrande et al (2010, A&A, 512, 54; CRMBA). Those reported byVandenBerg & Clem (2003, AJ, 126, 778) have also been considered, mainly toresolve some outstanding questions concerning them. Remarkably, when thesubdwarfs in the CRMBA data set that have sigma_pi/pi < 0.15 are superimposedon a set of 12 Gyr isochrones spanning a wide range in [Fe/H], the inferredmetallicities and effective temperatures agree, in the mean, with those givenby CRMBA to within +/- 0.05 dex and +/- 10 K, respectively. Thus the hot Teffscale derived by CRMBA is nearly identical with that predicted by stellarmodels and consequently, there is excellent consistency between theory andobservations on the H-R diagram and the different color-magnitude diagramsconsidered in this investigation. To obtain similar consistency, the colorsobtained from the MARCS and VandenBerg & Clem B-V vs. Teff relations formetal-poor dwarf stars should be adjusted to the red by 0.02-0.03 mag. Ingeneral, isochrones that employ the CRMBA transformations provide reasonablygood fits to our BV(RI)_C photometry for main-sequence stars in the globularclusters 47 Tuc, M3, M5, M92 and NGC 1851 - but not the cluster giants (whenadopting the synthetic MARCS colors). We speculate that differences between theactual heavy-element mixtures and those assumed in the theoretical models maybe the primary cause of this difficulty.

An Examination of Recent Transformations to the BV(RI)_C Photometric System from the Perspective of Stellar Models for Old Stars