Top: spectral coverage of the quasar composite spectrum by two adjacent photometric bands k,l (k = u,g,v,r and l = g,v,r,i,z) as a function of redshift z. The next rows (top to bottom) show the corresponding colour indices m_k − m_l, the variability ratios Q_k,l, and the gradients dlog Q_k,l/dM_i, respectively. Polygons: medians of colour index and log Q_k,l, respectively.