We introduce and discuss the properties of a theoretical ( B - K ) - ( J - K ) integrated colour diagram for single-age , single-metallicity stellar populations .
We show how this combination of integrated colours is able to largely disentangle the well known age-metallicity degeneracy when the age of the population is greater than \sim 300 Myr , and thus provides valuable estimates of both age and metallicity of unresolved stellar systems .
We discuss in detail the effect on this colour-colour diagram of \alpha -enhanced metal abundance ratios ( typical of the oldest populations in the Galaxy ) , the presence of blue horizontal branch stars unaccounted for in the theoretical calibration , and of statistical colour fluctuations in low mass stellar systems .
In the case of populations with multiple stellar generations , the luminosity-weighted mean age obtained from this diagram is shown to be heavily biased towards the youngest stellar components .
We then apply this method to several datasets for which optical and near-IR photometry are available in the literature .
We find that LMC and M31 clusters have colours which are consistent with the predictions of the models , but these do not provide a sensitive test due to the fluctuations which are predicted by our modelling of the Poisson statistics in such low-mass systems .
For the two Local Group dwarf galaxies NGC185 and NGC6822 , the mean ages derived from the integrated colours are consistent with the star formation histories inferred independently from photometric observations of their resolved stellar populations .

The methods developed here are applied to samples of nearby early-type galaxies with high quality aperture photometry in the literature .
A sample of bright field and Virgo cluster elliptical galaxies is found to exhibit a range of luminosity-weighted mean ages from 3 to 14 Gyr , with a mean of \sim 8 Gyr , independent of environment , and mean metallicities at or just above the solar value .
Colour gradients are found in all of the galaxies studied , in the sense that central regions are redder .
Apart from two radio galaxies , where the extreme central colours are clearly driven by the AGN , and one galaxy which also shows a radial age gradient , these colour changes appear consistent with metallicity changes at a constant mean age .
Finally , aperture data for five Virgo early-type dwarf galaxies shows that these galaxies appear to be shifted to lower mean metallicities and lower mean ages ( range 1 to 6 Gyr ) than their higher luminosity counterparts .