Colour indices, m_k − m_l, (top) and variability ratios, Q_k,l, (second row) as a function of redshift and variability index, V_k, as a function of absolute magnitude, M_i, (third row) for radio-loud quasars (dots) and quasars with unusual spectra (squares and arrows), respectively. Curves and hatched areas as in Fig. 8, filled squares in the third row: unusual quasars with weak emission lines (Fig. 11, left panel). Bottom: normalized distributions of log V_k for the unusual quasars (open squares and dashed lines), the radio-loud quasars (dots and dotted lines), and the standard sample (solid).