  (Abridged) This paper presents integrated magnitude and colours for synthetic
clusters. The integrated parameters have been obtained for the whole cluster
population as well as for the main-sequence (MS) population of star clusters.
We have also estimated observed integrated magnitudes and colours of MS
population of galactic open clusters, LMC and SMC star clusters. It is found
that the colour evolution of MS population of star clusters is not affected by
the stochastic fluctuations, however these fluctuations significantly affect
the colour evolution of the whole cluster population. The fluctuations are
maximum in $(V-I)$ colour in the age range 6.7 $<$ log (age) $<$ 7.5. Evolution
of integrated colours of MS population of the clusters in the Milky Way, LMC
and SMC, obtained in the present study are well explained by the present
synthetic cluster model. The observed integrated $(B-V)$ colours of MS
population of LMC star clusters having age $\geq$ 500 Myr seem to be
distributed around $Z=$ 0.004 model, whereas $(V-I)$ colours are found to be
more bluer than those predicted by the $Z=$ 0.004 model. $(V-I)$ vs $(B-V)$
two-colour diagram for the MS population of the Milky Way star clusters shows a
fair agreement between the observations and present model, however the diagrams
for LMC and SMC clusters indicate that observed $(V-I)$ colours are relatively
bluer. Possible reasons for this anomaly have been discussed.
