Evolution of cluster colour in the optical regime and the effects of M_c (upper row) and m_l (bottom row). The curve in all panels shows the continuous-IMF model, the connected symbols show the discrete-IMF case. In the upper row we compare M_c = 5  ×  10²   m_⊙ (left) and 10⁴   m_⊙ (right) cases for m_l = 0.1   m_⊙. The lower row is constructed for M_c = 10⁴   m_⊙ and m_l = 0.01   m_⊙ (left) and 1   m_⊙ (right).