  We discuss the systematic uncertainties inherent to analyses of observed
(broad-band) Spectral Energy Distributions (SEDs) of star clusters with
evolutionary synthesis models. We investigate the effects caused by restricting
oneself to a limited number of available passbands, choices of various passband
combinations, finite observational errors, non-continuous model input parameter
values, and restrictions in parameter space allowed during analysis. Starting
from a complete set of UBVRIJH passbands (respectively their Hubble Space
Telescope/WFPC2 equivalents) we investigate to which extent clusters with
different combinations of age, metallicity, internal extinction and mass can or
cannot be disentangled in the various evolutionary stages throughout their
lifetimes and what are the most useful passbands required to resolve the
ambiguities. We find the U and B bands to be of the highest significance, while
the V band and near-infrared data provide additional constraints. A code is
presented that makes use of luminosities of a star cluster system in all of the
possibly available passbands, and tries to find ranges of allowed
age-metallicity-extinction-mass combinations for individual members of star
cluster systems. Numerous tests and examples are presented. We show the
importance of good photometric accuracies and of determining the cluster
parameters independently without any prior assumptions.
