  To assess the current status in the theoretical modeling of the spectral
properties of late-type giants, we provide a comparison of synthetic
photometric colors of late-type giants (calculated with PHOENIX, MARCS and
ATLAS model atmospheres) with observations, at [M/H]=0.0 and -2.0. Overall,
there is a good agreement between observed and synthetic colors, and synthetic
colors and published Teff-color relations, both at [M/H]=0.0 and -2.0.
Deviations from the observed trends in Teff-color planes are generally within
\pm 150K (or less) in the effective temperature range Teff=3500-4800K.
Synthetic colors calculated with different stellar atmosphere models typically
agree to ~100K, within a large range of effective temperatures and gravities.
Some discrepancies are seen in the Teff-(B-V) plane below Teff~3800K at
[M/H]=0.0, due to difficulties in reproducing the 'turn-off' to the bluer
colors which is seen in the observed data at Teff~3600K. Note that at
[M/H]=-2.0 effective temperatures given by the scale of Alonso et al. (1999)
are generally lower than those resulting from other Teff-color relations based
both on observed and synthetic colors.
