We have calculated extensive grids of magnitudes and colours for stars of
any spectral type in several pass-bands of the HST WFPC2 and FOC,
as well as in the
Johnson-Cousins system. The analysis has been made using the library
of stellar spectra calculated by Kurucz (1992) and filtering the
spectral energy distribution with the convolution of the pass-band and
quantum efficiency of the WFPC2 and/or FOC and the telescope assembly.
The HST magnitudes
are presented by means of
tabulations of Bolometric Corrections
(
)
as a function
of the metallicity, 
, and gravity which allow us to
convert the bolometric luminosity of a star into the HST magnitudes
and colours.
We have also presented integrated magnitudes and colours as a function of the age for SSPs obeying the Salpeter initial mass function.
Furthermore, we have analyzed the CMD of a typical sample of stars
containing both
old and young objects. The results are presented both for the
Johnson-Cousins
and the HST-WFPC2 photometric systems pointing out the distortion
engendered in
this latter by the non monotonic colour-
relation for the
extreme UV pass-bands. These simulations are
intended to call the reader attention
on the peculiarity inherent to the UV pass-bands
and on the unexpected appearance of the otherwise familiar
CMDs in those HST UV pass-bands.
Finally, we have presented the colour evolution of elliptical galaxies as a
function of the red-shift in order to check whether the onset of the P-AGB
and/or
H-HB and AGB-manqué stars as old sources of UV radiation can be
detected in
suitable UV colours at certain values of the red-shift. We find that
the colour -red-shift relations exhibit signatures that can be traced back to
particular evolutionary stages of the underlying stellar populations, whose
nature and age are known at certain degree of confidence. Furthermore
the detailed colour-red-shift relation is found to depend also on the
cosmological parameters. If these predictions are confirmed by
observational data they will allow us to infer
the type of source of UV radiation, and constrain the galaxy age and
cosmological parameters 
and 
.
Detailed tabulations of isochrones and 
for other chemical compositions and/or pass-bands are available from the
authors upon request.
Acknowledgements
This work was supported by Italian Space Agency (ASI) and the Ministry of University, Scientific Research, and Technology of Italy (MURST).