  We aim to test the power of theoretical calibrations based on a new
generation of MARCS models by comparisons with observational photomteric data.
We calculate synthetic uvby-Hbeta colour indices from synthetic spectra. A
sample of 388 field stars as well as stars in globular clusters is used for a
direct comparison of the synthetic indices versus empirical data and for
scrutinizing the possibilities of theoretical calibrations for temperature,
metallicity and gravity. We show that the temperature sensitivity of the
synthetic (b-y) colour is very close to its empirical counterpart, whereas the
temperature scale based upon Hbeta shows a slight offset. The theoretical
metallicity sensitivity of the m1 index (and for G-type stars its combination
with c1) is somewhat larger than the empirical one, based upon spectroscopic
determinations. The gravity sensitivity of the synthetic c1 index shows a
satisfactory behaviour when compared to obervations of F stars. For stars
cooler than the sun a deviation is significant in the c1-(b-y) diagram. The
theoretical calibrations of (b-y), (v-y) and c1 seem to work well for Pop II
stars and lead to effective temperatures for globular cluster stars supporting
recent claims by Korn et al. (2007) that atomic diffusion occurs in stars near
the turnoff point of NGC 6397. Synthetic colours of stellar atmospheres can
indeed be used, in many cases, to derive reliable fundamental stellar
parameters. The deviations seen when compared to observational data could be
due to incomplete linelists but are possibly also due to effects of assuming
plane-parallell or spherical geometry and LTE.
