MIUSCAT: extended MILES spectral coverage. II. Constraints from optical
  photometry
  In the present work we show a comprehensive comparison of our new stellar
population synthesis MIUSCAT models with photometric data of globular clusters
and early-type galaxies. The models compare remarkably well with the colours of
Milky Way globular clusters in the optical range. Likewise, the colours of M31
globular clusters can also be explained by the models by assuming younger ages
then their Galactic counterparts. When compared with quiescent galaxies we
reproduce the colour evolution at intermediate redshift. On the other hand we
find that the colour relations of nearby early-type galaxies are still a
challenge for present-day stellar population synthesis models. We investigate a
number of possible explanations and establish the importance of alpha-enhanced
models to bring down the discrepancy with observations.
