  We have performed the first extensive analysis of the impact of an
[alpha/Fe]>0 metal distribution on broadband colors in the parameter space
(surface gravity, effective temperature, metal content) covered by Galactic
globular cluster stars.A comparison of updated and homogeneous ATLAS9 UBVRIJHKL
synthetic photometry, for both alpha-enhanced and scaled-solar metal
distributions, has shown that it is impossible to reproduce alpha-enhanced
(B-V) and (U-B) color transformations with simple rescalings of the
scaled-solar ones. At [Fe/H]$\sim -$2.0 alpha-enhanced transformations are well
reproduced by scaled-solar ones with the same [Fe/H], but this good agreement
breaks down at [Fe/H] larger than about -1.6.As a general rule, (B-V) and (U-B)
alpha-enhanced colors are bluer than scaled-solar ones at either the same
[Fe/H] or [M/H], and the differences increase with increasing metallicity and
decreasing T_eff. A preliminary analysis of the contribution of the various
alpha elements to the stellar colors shows that the magnesium abundance (and to
lesser extent oxygen and silicon) is the main responsible for these
differences. On the contrary, the bolometric correction to the V band and more
infrared colors predicted by alpha-enhanced transformations are well reproduced
by scaled-solar results, due to their weak dependence on the metal content.Key
parameters like the Turn Off and Zero Age Horizontal Branch V magnitudes, as
well as the red Giant Branch tip I magnitude obtained from theoretical
isochrones are in general unaffected when using the appropriate alpha-enhanced
transformations in place of scaled-solar ones. We have also studied the effect
of boundary conditions obtained from appropriate alpha-enhanced model
atmospheres on the evolutionary tracks in the HR diagram.
