  Buzzoni's (1989) grid of synthetic spectral energy distributions,
representative of old stellar populations, was used to derive colours in
different photometric systems, and to compare the theoretical predictions with
the observational data referring to about $120$ globular clusters in the Galaxy
and to $159$ objects of the globular cluster system of M31. Synthetic and
observed indices display an overall agreement in the composite planes of
two-colour diagrams, thus in agreement with the standard evolutionary scenario
leading, for globular clusters, to old stellar populations consistent with an
age of $15$ Gyr and a Salpeter initial mass function (IMF). The two main
parameters modulating the cluster colour distributions are, as known,
metallicity and horizontal branch morphology, while IMF slope and mass loss
rate from stars in the red-giant branch and asymptotic-giant branch
evolutionary stages produce only minor, although not negligible, effects on the
integrated colours. The M31 and Galactic cluster populations are found to be
substantially similar, at least as far as the spectral energy distribution
characteristics in the $U \to V$ spectral range are concerned.
