Broad-band colour evolution of star clusters

We briefly review the main features in the broad-band colour evolution of star clusters, over the complete age interval from 10 to 10 yr. The emphasis is in the problem of age-dating distant young clusters (∼< 2 Gyr) from their integrated colours. It is shown that U−B and B−V are less sensitive to metallicity than colours involving red passbands, like V −I, at least up to ages of some few Gyr. Since U−B and B−V are determined by well-understood and well-populated evolutionary stages, they are also less affected by theoretical uncertainties and by the ubiquitous effect of stochastic colour dispersion. The latter effects become important for the V −K colour. Thus, we argue that U−B and B−V are, presently, the more suitable broad-band colours for age-dating distant clusters. For other potentially useful colours like V −R and V −I, empirical tests of their evolution are still missing.