In this work, we show a comprehensive comparison of our new stellar population synthesis (SPS) MIUSCAT models with photometric data of globular clusters (GCs) and early-type galaxies.The models compare remarkably well with the colours of Milky Way GCs in the optical range. Likewise, the colours of M31 GCs can also be explained by the models by assuming younger ages than 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 SPS models. We investigate a number of possible explanations and establish the importance of α-enhanced models to bring down the discrepancy with observations.