Integrated magnitudes of open clusters in two different passbands vs. cluster age. The observations of the Galactic open clusters are shown 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 indicate typical errors of the integrated magnitudes. The curves are evolutionary tracks of a continuous model with effects of mass loss incorporated for model parameters M_c = 10³   m_⊙, and m_l = 0.01   m_⊙ (solid curve), and M_c = 5  ×  10²   m_⊙, m_l = 0.1   m_⊙ (dash-dotted curve). The latter option (M_c = 5  ×  10²   m_⊙, m_l = 0.1   m_⊙), but with discrete IMF, is shown by the connected crosses.