  Stellar models for -2.5 <= [Fe/H] <= -0.5 that have been computed for
variations in the C:N:O abundance ratio (for two different values of [CNO/Fe])
are compared with HST Wide Field Camera 3 (WFC3) observations of the globular
clusters (GCs) 47 Tuc, NGC 6362, M5, M5, M55, and M92. The bolometric
corrections used to transpose the models to the observed planes are based on
new MARCS synthetic spectra that incorporate improved treatments of molecules
that involve atoms of C, N, and O. On the assumption of well-supported distance
moduli and reddenings, isochrones for [O/Fe] = 0.6 and [m/Fe] = 0.4 for the
other alpha elements, which are favoured by binary stars in GCs, generally
reproduce the main features of observed colour-magnitude diagrams (CMDs) to
within ~ 0.03 mag. In particular, they appear to match the spreads in the
observed (M_F336W - M_F438W)_0 colours that are spanned by CN-weak and
CN-strong stars along the lower giant branch quite well, but not the bluest
giants, which are suspected to be N-poor ([N/Fe] <~ -0.5). Both the absolute
(M_F438W - M_F606W)_0 colours and the variations in these colours at a given
M_F606W magnitude on the giant branch are difficult to explain unless the
reddest stars are C-rich ([C/Fe] >~ +0.5). Allowing for moderate He abundance
variations (delta Y ~ 0.05) improves the fits to the observations.
