  We present a combined study of the colour-magnitude relation, colour
distribution and luminosity function of a sample of 24 clusters at redshifts
0.3<z<1. The sample is largely composed of X-ray selected/detected clusters.
Most of the clusters at redshifts z<0.6 display X-ray luminosity or richness
typical of poor clusters or groups, rather than the more typical, massive
clusters studied in literature at redshifts z>~0.3. All our clusters, including
groups, display a colour-magnitude relation consistent with a passively
evolving stellar population formed at a redshift z_f>~2, in accordance with
observed galaxy populations in more massive clusters studied at comparable
redshifts. Colours and luminosity functions show that the cluster galaxy
population is consistent with the presence of at least two components: old
systems formed at high redshift that have evolved passively from that epoch,
together with a galaxy population displaying more recent star formation. The
former population forms at ~2< z_f <~5, the latter at redshifts z<1. A model in
which stars do not evolve is clearly rejected by both by the colour of reddest
galaxies and by the characteristic luminosity m* measures. All clusters (with
one possible exception) are detected independently by an almost three
dimensional optical search employing sky position and colour - this despite the
primary X-ray selection and low X-ray flux/optical richness displayed by the
majority of the sample.
