Integrated colours of open clusters vs. cluster age. The left panel shows data for (B − V), the right one for (J − K_s). The observations of the Galactic open clusters are indicated by dots, blue for MS-clusters, green for post-MS clusters, and red for RG-clusters (see Sect. 2 for explanation). Bars in the upper-right corner of each panel mark typical errors of the integrated colours. The solid curves are evolutionary tracks of a SSP model with a continuous IMF for a cluster with M_c = 10³   m_⊙ and m_l = 0.1   m_⊙. The connected symbols (crosses) show the discrete-IMF option of this model.